feat(#1945): tracer-first planning default + executor feedback gate (#2294)

* feat(#1945): tracer-first planning default + executor feedback gate

Make "thin end-to-end slice first, verify, then expand" the default planning + execution discipline instead of the opt-in --mvp mode.

- gsd-planner: first-class `type="tracer"` task; every plan LEADS with one production-quality end-to-end tracer slice by default; --no-tracer restores horizontal layers; --mvp/--tdd compose on top.
- gsd-executor + execute-plan: post-tracer feedback gate — autonomous runs halt-on-fail before expansion, interactive runs emit checkpoint:human-verify after the tracer.
- --no-tracer flag wired through plan-phase workflow/command/help/skill.
- CONTEXT.md glossary defines tracer bullet vs prototype; docs + references reconciled.
- tests/tracer-bullet.test.cjs: prose-contract + behavioral (verify plan-structure accepts tracer) coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#1945): backfill changeset PR number to 2294

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

gsd-core/workflows/execute-plan.md

@@ -189,6 +189,7 @@ Deviations are normal — handle via rules below.
 3. Per task:
    - **MANDATORY read_first gate:** If the task has a `<read_first>` field, you MUST read every listed file BEFORE making any edits. This is not optional. Do not skip files because you "already know" what's in them — read them. The read_first files establish ground truth for the task.
    - `type="auto"`: if `tdd="true"` → TDD execution. Implement with deviation rules + auth gates. Verify done criteria. Commit (see task_commit). Track hash for Summary.
+   - `type="tracer"`: execute like `type="auto"` (production-quality, real `<verify>`, commit), then run the tracer feedback gate BEFORE any expansion task — an early integration checkpoint. Auto mode active (`AUTO_CHAIN` or `AUTO_CFG`): re-run the tracer `<verify>`; on failure HALT and surface (deviation) — do NOT start expansion tasks. Interactive: STOP → return a `checkpoint:human-verify` for the tracer via checkpoint_protocol before expansion.
    - `type="checkpoint:*"`: STOP → checkpoint_protocol → wait for user → continue only after confirmation.
    - **HARD GATE — acceptance_criteria verification:** After completing each task, if it has `<acceptance_criteria>`, you MUST run a verification loop before proceeding:
      1. For each criterion: execute the grep, file check, or CLI command that proves it passes

gsd-core/workflows/help/modes/full.md

@@ -105,7 +105,7 @@ Usage: `/gsd:discuss-phase 2`
 Usage: `/gsd:discuss-phase 2 --batch`
 Usage: `/gsd:discuss-phase 2 --batch=3`
 
-**`/gsd:plan-phase <number> [--research] [--skip-research] [--research-phase <N>] [--view] [--gaps] [--skip-verify] [--prd <file>] [--ingest <path-or-glob>] [--ingest-format <auto|nygard|madr|narrative>] [--reviews] [--text] [--tdd] [--mvp]`**
+**`/gsd:plan-phase <number> [--research] [--skip-research] [--research-phase <N>] [--view] [--gaps] [--skip-verify] [--prd <file>] [--ingest <path-or-glob>] [--ingest-format <auto|nygard|madr|narrative>] [--reviews] [--text] [--tdd] [--mvp] [--no-tracer]`**
 Create detailed execution plan for a specific phase.
 
 - `--skip-research` — bypass the research subagent
@@ -116,7 +116,8 @@ Create detailed execution plan for a specific phase.
 - `--ingest <path-or-glob>` — pre-ingest external ADRs/PRDs/SPECs before planning (see *PRD Express Path* below)
 - `--ingest-format <auto|nygard|madr|narrative>` — hint the ADR ingester's parser when `--ingest` is set; defaults to `auto`
 - `--tdd` — plan in test-driven order (tests before code)
-- `--mvp` — vertical-slice MVP planning mode (see also `/gsd:mvp-phase`)
+- `--mvp` — MVP enrichment (user story + Walking Skeleton) on top of the default tracer-first ordering (see also `/gsd:mvp-phase`)
+- `--no-tracer` — opt out of the default tracer-first slice and plan horizontal layers (legacy default)
 
 - Generates `.planning/phases/XX-phase-name/XX-YY-PLAN.md`
 - Breaks phase into concrete, actionable tasks

gsd-core/workflows/plan-phase.md

@@ -95,7 +95,7 @@ Read and execute `gsd-core/workflows/plan-phase/steps/closed-phase-gate.md` —
 
 ## 2. Parse and Normalize Arguments
 
-Extract from $ARGUMENTS: phase number (integer or decimal like `2.1`), flags (`--research`, `--skip-research`, `--research-phase <N>`, `--gaps`, `--skip-verify`, `--skip-ui`, `--prd <filepath>`, `--ingest <path-or-glob>`, `--ingest-format <auto|nygard|madr|narrative>`, `--reviews`, `--text`, `--bounce`, `--skip-bounce`, `--chunked`, `--mvp`, `--tdd`, `--granularity <coarse|standard|fine>`, `--force` (override closed-phase gate, see §1.5)).
+Extract from $ARGUMENTS: phase number (integer or decimal like `2.1`), flags (`--research`, `--skip-research`, `--research-phase <N>`, `--gaps`, `--skip-verify`, `--skip-ui`, `--prd <filepath>`, `--ingest <path-or-glob>`, `--ingest-format <auto|nygard|madr|narrative>`, `--reviews`, `--text`, `--bounce`, `--skip-bounce`, `--chunked`, `--mvp`, `--no-tracer`, `--tdd`, `--granularity <coarse|standard|fine>`, `--force` (override closed-phase gate, see §1.5)).
 
 **`--research-phase <N>` — research-only mode (#3042 + #3044).** When this flag is present, parse `<N>` as the phase number (overrides any positional phase argument), set `RESEARCH_ONLY=true`, and treat the rest of this workflow as a research-dispatch only — the planner spawn (step 8), plan-checker, verification, gaps, bounce, and post-planning-gaps blocks all skip on `RESEARCH_ONLY`. Use this for cross-phase research, doc review before committing to a planning approach, and correction-without-replanning loops. Replaces the deleted `/gsd-research-phase` command.
 
@@ -128,8 +128,13 @@ if [[ "$ARGUMENTS" =~ (^|[[:space:]])--mvp([[:space:]]|$) ]]; then MVP_FLAG_ARG=
 if [[ "$ARGUMENTS" =~ (^|[[:space:]])--tdd([[:space:]]|$) ]]; then
   gsd_run query config-set workflow.tdd_mode true 2>/dev/null || true
 fi
+# Tracer-first is the default; --no-tracer opts back into the legacy horizontal-layer shape.
+TRACER_MODE=true
+if [[ "$ARGUMENTS" =~ (^|[[:space:]])--no-tracer([[:space:]]|$) ]]; then TRACER_MODE=false; fi
 ```
 
+**Tracer-first resolution.** `TRACER_MODE` defaults to `true` — every plan LEADS with one `type="tracer"` end-to-end slice, then expansion tasks. `--no-tracer` sets `TRACER_MODE=false` to restore the legacy horizontal-layer default. Unlike `MVP_MODE`, tracer-first is not a persisted per-phase mode — it is the baseline decomposition discipline, so there is no roadmap/config chain to consult.
+
 Defer the `phase.mvp-mode` query until `PHASE` is finalized (after explicit argument parsing/fallback phase detection + validation). The verb returns `true|false`; full result also exposes `source` (`cli_flag` | `roadmap` | `config` | `none`) for diagnostics. Mode is **all-or-nothing per phase** (PRD decision Q1).
 
 **Walking Skeleton gate.** When `MVP_MODE=true` AND `phase_number == "01"` AND there are zero prior phase summaries (new project), the planner runs in **Walking Skeleton mode** (per PRD decision Q2 — new projects only). Detect with:
@@ -777,6 +782,7 @@ Historical findings already incorporated, explicitly deferred/rejected in PLAN.m
 
 {For each active entry in `PLAN_PRE_HOOKS_JSON` where `kind == "contribution"` and `into == "planner"` (in array order): inject the entry's `fragment.inline` verbatim here. This delivers all planner-targeted contributions — including tdd's `<tdd_mode_active>` block (type:tdd heuristics), schema-gate's schema-push detection guidance (if active at plan:pre), and security's threat-model guidance. For the security contribution, also surface the resolved `configValues`: `security_asvs_level` (ASVS enforcement level) and `security_block_on` (severity threshold) so the planner uses the configured values when generating `<threat_model>` blocks. If no active planner contributions exist, omit this block entirely.}
 
+**TRACER_MODE:** ${TRACER_MODE} (when true — the default — the plan LEADS with one `type="tracer"` end-to-end slice touching every layer, then expansion tasks; when false (`--no-tracer`), decompose into horizontal layers.)
 **MVP_MODE:** ${MVP_MODE} (when true, follow vertical-slice rules from `~/.claude/gsd-core/references/planner-mvp-mode.md`; when false, ignore MVP guidance entirely.)
 **WALKING_SKELETON:** ${WALKING_SKELETON} (when true, the first deliverable must be a Walking Skeleton — Read the template at `~/.claude/gsd-core/references/skeleton-template.md` and produce SKELETON.md alongside PLAN.md.)
 **Granularity:** {granularity}