feat: requirements coverage gate in plan-phase pipeline (#984) (#1121)

Add step 13 (Requirements Coverage Gate) to plan-phase workflow.
After plans pass the checker, verifies all phase requirements are
covered by at least one plan before declaring planning complete.

- Extracts REQ-IDs from plan frontmatter and compares against
  phase_req_ids from ROADMAP
- Cross-checks CONTEXT.md features against plan objectives to
  detect silently dropped scope
- Reports gaps with options: re-plan, defer, or proceed
- Skips when phase_req_ids is null/TBD (no requirements mapped)

Fixes #984

Co-authored-by: TÂCHES <afromanguy@me.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   - Pre-wave dependency check in `execute-phase`: verifies key-links from prior wave artifacts before spawning next wave
   - Cross-Plan Data Contracts (Dimension 9) in plan-checker: detects incompatible transformations between plans sharing data pipelines
   - Export-level spot check in `verify-phase`: catches dead stores that exist in wired files but are never called
+- **Requirements coverage gate in plan-phase** — New step 13 verifies all phase requirements are covered by at least one plan before planning completes. Cross-checks REQ-IDs from ROADMAP against plan frontmatter and CONTEXT.md features against plan objectives (#984)
 - **`/gsd:ship` command** — Native PR creation workflow that bridges local completion → merged PR. Auto-generates rich PR body from planning artifacts (SUMMARY.md, VERIFICATION.md, REQUIREMENTS.md), pushes branch, creates PR via `gh`, optionally requests review, and updates STATE.md with shipping status (#829)
 
 ### Fixed