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