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>

tests/workflow-size-baseline.json

@@ -25,7 +25,7 @@
   "edit-phase.md": 12927,
   "eval-review.md": 9967,
   "execute-phase.md": 93583,
-  "execute-plan.md": 32655,
+  "execute-plan.md": 33113,
   "explore.md": 10541,
   "extract-learnings.md": 12893,
   "fast.md": 7613,
@@ -53,7 +53,7 @@
   "onboard.md": 8590,
   "pause-work.md": 14441,
   "plan-milestone-gaps.md": 11809,
-  "plan-phase.md": 93113,
+  "plan-phase.md": 93959,
   "plan-review-convergence.md": 23512,
   "plant-seed.md": 11785,
   "pr-branch.md": 15963,