Adds a new slash command that lets developers modify any field of an
existing phase in ROADMAP.md without affecting phase number or position.
- commands/gsd/edit-phase.md: command file with --force flag support
- get-shit-done/workflows/edit-phase.md: full workflow with status guard,
depends_on validation, diff+confirmation, and STATE.md update
- tests/edit-phase.test.cjs: 32 tests covering all acceptance criteria
- docs/INVENTORY.md, INVENTORY-MANIFEST.json, COMMANDS.md: registered
Closes#2617
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
