feat(#429): HARD GATE on negative-grep literals echoed in plan <action> bodies (#1062)

Convert the planner's soft comment-text guideline into a plan-write-time
HARD GATE. When an acceptance criterion negative-greps for a literal
(`grep -c 'LIT' file == 0`) and that same literal appears verbatim in an
`<action>` body (JSDoc samples, head-comment references, "what NOT to do"
snippets), the executor's commit-time verify gate later fails on the
comment echo rather than a real regression — wasting cycles and training
the executor to distrust the gate.

`verify.plan-structure` (the `validate_plan` step) now scans for this:
- confidently-extracted (quoted) negative-grep literal echoed in an
  <action> → error (valid:false), failing plan creation
- unquoted/ambiguous grep target → warning (fallback policy)
- `<!-- planner-discipline-allow: LIT -->` escape hatch skips a literal
- positive-count gates (`== N`) and `!= 0`/`>= 0` are out of scope

Adds the `<comment_text_discipline>` block to gsd-planner.md, the full
rules + allowlist example to planner-antipatterns.md, and regression
fixtures for downstream incidents 12-04, 11-04, 12-02 (plus a boundary
case proving positive-count gate 11-02 is not flagged).

Closes #429

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/AGENTS.md

@@ -173,6 +173,7 @@ GSD uses a multi-agent architecture where thin orchestrators (workflow files) sp
 - Includes `read_first` and `acceptance_criteria` sections
 - Groups plans into dependency waves
 - Performs reachability check to validate plan steps reference accessible files and APIs (v1.32)
+- Enforces a comment-text discipline HARD GATE at plan-write time (`verify.plan-structure`): a literal that an acceptance criterion negative-greps for (`grep -c 'LIT' file == 0`) must not appear verbatim in an `<action>` body; violations fail plan creation. Use `<!-- planner-discipline-allow: LIT -->` to allowlist a legitimate occurrence. (#429)
 
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