feat: /gsd:ship command for PR creation from verified phase work (#829) (#1123)

* feat: /gsd:ship command for PR creation from verified phase work (#829)

New command that bridges local completion → merged PR, closing the
plan → execute → verify → ship loop.

Workflow (workflows/ship.md):
1. Preflight: verification passed, clean tree, correct branch, gh auth
2. Push branch to remote
3. Auto-generate rich PR body from planning artifacts:
   - Phase goal from ROADMAP.md
   - Changes from SUMMARY.md files
   - Requirements addressed (REQ-IDs)
   - Verification status
   - Key decisions
4. Create PR via gh CLI (supports --draft)
5. Optional code review request
6. Update STATE.md with shipping status

Files:
- commands/gsd/ship.md: New command entry point
- get-shit-done/workflows/ship.md: Full workflow implementation
- get-shit-done/workflows/help.md: Add ship to help output
- docs/COMMANDS.md: Command reference
- docs/FEATURES.md: Feature spec with REQ-SHIP-01 through 05
- docs/USER-GUIDE.md: Add to command table
- CHANGELOG.md: Document new command

Fixes #829

* fix(tests): update expected skill count from 39 to 40 for new ship command

The Copilot install E2E tests hardcode the expected number of skill
directories and manifest entries. Adding commands/gsd/ship.md increased
the count from 39 to 40.

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
+- **`/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
 - **Requirements `mark-complete` is now idempotent** — Re-marking already-completed requirements returns `already_complete` instead of `not_found` (#948)