feat: implement --reviews flag for gsd:plan-phase

Wire the --reviews flag through the full stack so plan-phase can
replan incorporating cross-AI review feedback from REVIEWS.md:

- core.cjs: add has_reviews detection in searchPhaseInDir
- init.cjs: wire has_reviews and reviews_path through all init functions
- plan-phase.md command: add --reviews to argument-hint and flags
- plan-phase.md workflow: add step 2.5 validation, skip research,
  skip existing plans prompt, pass reviews_path to planner
- gsd-planner.md: add reviews_mode section for consuming review feedback
- COMMANDS.md: add --reviews and missing flags to docs

Closes the gap where --reviews was referenced in 6 places (review
workflow, review command, help workflow, COMMANDS.md, FEATURES.md)
but never implemented.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/COMMANDS.md

@@ -86,8 +86,12 @@ Research, plan, and verify a phase.
 | Flag | Description |
 |------|-------------|
 | `--auto` | Skip interactive confirmations |
+| `--research` | Force re-research even if RESEARCH.md exists |
 | `--skip-research` | Skip domain research step |
+| `--gaps` | Gap closure mode (reads VERIFICATION.md, skips research) |
 | `--skip-verify` | Skip plan checker verification loop |
+| `--prd <file>` | Use a PRD file instead of discuss-phase for context |
+| `--reviews` | Replan with cross-AI review feedback from REVIEWS.md |
 
 **Prerequisites:** `.planning/ROADMAP.md` exists
 **Produces:** `{phase}-RESEARCH.md`, `{phase}-{N}-PLAN.md`, `{phase}-VALIDATION.md`