feat(graceful-handoff): initial plugin with /graceful-handoff command

New plugin that produces a complete session handoff in under 60s:
NEXT-SESSION artifact, commit+push, and copy-paste prompt for next
session. Built for context-constrained models like Opus 4.7 where
sessions fill fast.

- Single declarative command, no hooks/agents/skills
- Detects handoff type: multi-session / plugin-work / single-task
- Default filename NEXT-SESSION-PROMPT.local.md; slug-override
- Flags: --no-commit, --dry-run
- Auto-generated Conventional Commits message from git diff --stat
- Respects pre-commit hooks (secrets, pathguard) — never bypasses

Also: add *.local.md to root .gitignore (existing NEXT-SESSION files
were untracked but not ignored) and list plugin in marketplace
README + CLAUDE.md per docs-convention.

.gitignore

@@ -2,6 +2,7 @@
 REMEMBER.md
 TODO.md
 ROADMAP.md
+*.local.md
 
 # Per-plugin session directories (plans, research, execution progress)
 plugins/*/.claude/