feat: add /gsd-edit-phase command to modify roadmap phases in place (#2753)

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>

docs/INVENTORY-MANIFEST.json

@@ -56,6 +56,7 @@
       "/gsd-discuss-phase",
       "/gsd-do",
       "/gsd-docs-update",
+      "/gsd-edit-phase",
       "/gsd-eval-review",
       "/gsd-execute-phase",
       "/gsd-explore",
@@ -145,6 +146,7 @@
       "discuss-phase.md",
       "do.md",
       "docs-update.md",
+      "edit-phase.md",
       "eval-review.md",
       "execute-phase.md",
       "execute-plan.md",