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

CHANGELOG.md

@@ -17,6 +17,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ### Fixed
 - **Requirements `mark-complete` is now idempotent** — Re-marking already-completed requirements returns `already_complete` instead of `not_found` (#948)
 
+### Improved
+- **Structured session handoff** — `/gsd:pause-work` now writes `.planning/HANDOFF.json` alongside `.continue-here.md`. JSON provides machine-readable state (task position, blockers, human actions pending, uncommitted files) that `/gsd:resume-work` parses for precise resumption instead of generic "what do you want to do?" (#940)
+
 ## [1.25.0] - 2026-03-16
 
 ### Added