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

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/tidy-goats-wake.md

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+---
+type: Changed
+pr: 2294
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+**Phase plans now lead with a verified end-to-end "tracer" slice by default** — every plan starts with one thin, production-quality slice wired through every layer, which the executor verifies before building out the remaining tasks, so an architectural dead-end surfaces after one commit instead of after ten. Pass `--no-tracer` to restore the previous horizontal-layer default; `--mvp` now layers user-story framing and the Walking Skeleton on top of the tracer-first ordering. (#1945)