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>

commands/gsd/edit-phase.md

@@ -0,0 +1,35 @@
+---
+name: gsd:edit-phase
+description: Edit any field of an existing roadmap phase in place, preserving number and position
+argument-hint: <phase-number> [--force]
+allowed-tools:
+  - Read
+  - Write
+  - Bash
+---
+
+<objective>
+Modify any field of an existing phase in ROADMAP.md in place.
+
+Supports:
+- Editing individual fields (title, description/goal, requirements, success criteria, depends_on)
+- Full regeneration of all fields from a clarified intent
+- Guarded edits: refuses in_progress/completed phases unless --force is passed
+- Depends-on validation: blocks invalid references with a clear error
+- Diff + confirmation before writing
+</objective>
+
+<execution_context>
+@~/.claude/get-shit-done/workflows/edit-phase.md
+</execution_context>
+
+<context>
+Arguments: $ARGUMENTS (format: <phase-number> [--force])
+
+Roadmap and state are resolved in-workflow via `init phase-op` and targeted reads.
+</context>
+
+<process>
+Execute the edit-phase workflow from @~/.claude/get-shit-done/workflows/edit-phase.md end-to-end.
+Preserve all validation gates (phase existence, status guard, depends_on validation, diff + confirmation).
+</process>