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/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