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>

docs/AGENTS.md

@@ -215,7 +215,8 @@ GSD uses a multi-agent architecture where thin orchestrators (workflow files) sp
 - Fresh 200K context window per plan
 - Follows XML task instructions precisely
 - Atomic git commit per completed task
-- Handles checkpoint types: auto, human-verify, decision, human-action
+- Handles task types: auto, tracer, checkpoint (human-verify, decision, human-action)
+- Tracer feedback gate: after a `tracer` slice, verifies it end-to-end before expansion tasks — autonomous runs halt on failure; interactive runs emit a human-verify checkpoint
 - Reports deviations from plan in SUMMARY.md
 - Invokes node repair on verification failure