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>

agents/gsd-executor.md

@@ -152,11 +152,17 @@ For each task:
    - Commit (see task_commit_protocol)
    - Track completion + commit hash for Summary
 
-2. **If `type="checkpoint:*"`:**
+2. **If `type="tracer"`:** (the leading thin end-to-end slice — production-quality, never a throwaway)
+   - Execute and commit exactly like `type="auto"` (real implementation, real `<verify>`, atomic commit).
+   - **Then run the tracer feedback gate BEFORE any expansion task** — an early integration checkpoint on the proven slice:
+     - **Autonomous run (auto mode active — `AUTO_CHAIN` or `AUTO_CFG` is `"true"`, per `<auto_mode_detection>`):** re-run the tracer's `<verify>` end-to-end. If it **fails**, HALT and surface it (deviation Rule 1) — do NOT proceed to expansion tasks. Pouring more layers onto a broken foundation is exactly the failure this gate prevents. If it passes, log `⚡ Tracer verified end-to-end — expanding` and continue.
+     - **Interactive run (auto mode not active):** immediately after committing the tracer, STOP and return a `checkpoint:human-verify` for the tracer's `<verify>` (the working slice) using checkpoint_return_format, before any expansion task.
+
+3. **If `type="checkpoint:*"`:**
    - STOP immediately — return structured checkpoint message
    - A fresh agent will be spawned to continue
 
-3. After all tasks: run overall verification, confirm success criteria, document deviations
+4. After all tasks: run overall verification, confirm success criteria, document deviations
 </step>
 
 </execution_flow>
@@ -310,6 +316,8 @@ For full automation-first patterns, server lifecycle, CLI handling:
 
 **Quick reference:** Users NEVER run CLI commands. Users ONLY visit URLs, click UI, evaluate visuals, provide secrets. Claude does all automation.
 
+**Tracer feedback gate:** a `type="tracer"` task is followed by an early integration checkpoint on the proven slice (see `<execution_flow>` → `execute_tasks`) — in autonomous runs a failing tracer `<verify>` HALTS before any expansion task; in interactive runs the executor emits a `checkpoint:human-verify` for the tracer immediately after committing it.
+
 ---
 
 **Auto-mode checkpoint behavior** (when `AUTO_CFG` is `"true"`):