feat: structured session handoff artifact for cross-session continuity (#940) (#1122)

Enhance /gsd:pause-work to write .planning/HANDOFF.json alongside
.continue-here.md. The JSON provides machine-readable state that
/gsd:resume-work can parse for precise resumption.

HANDOFF.json includes:
- Task position (phase, plan, task number, status)
- Completed and remaining tasks with commit hashes
- Blockers with type classification (technical/human_action/external)
- Human actions pending (API keys, approvals, manual testing)
- Uncommitted files list
- Context notes for mental model restoration

Resume-work changes:
- HANDOFF.json is primary resumption source (highest priority)
- Surfaces blockers and human actions immediately on session start
- Validates uncommitted files against git status
- Deletes HANDOFF.json after successful resumption
- Falls back to .continue-here.md if no JSON exists

Also checks for placeholder content in SUMMARY.md files to catch
false completions (frontmatter claims complete but body has TBD).

Fixes #940

docs/USER-GUIDE.md

@@ -296,7 +296,7 @@ Controlled by `workflow.ui_safety_gate` config toggle.
 |---------|---------|-------------|
 | `/gsd:progress` | Show status and next steps | Anytime -- "where am I?" |
 | `/gsd:resume-work` | Restore full context from last session | Starting a new session |
-| `/gsd:pause-work` | Save context handoff | Stopping mid-phase |
+| `/gsd:pause-work` | Save structured handoff (HANDOFF.json + continue-here.md) | Stopping mid-phase |
 | `/gsd:help` | Show all commands | Quick reference |
 | `/gsd:update` | Update GSD with changelog preview | Check for new versions |
 | `/gsd:join-discord` | Open Discord community invite | Questions or community |