harness: gitignore tmp/ + add promotion check to /save and /scc

- .gitignore: ignore root tmp/ (temp/ and .claude/tmp/ were already ignored;
  root tmp/ was not, which is how scratch got committed and needed cleanup).
- New .claude/scripts/tmp-promotion-check.sh: advisory, read-only, never blocks.
  Scans the gitignored scratch dirs (tmp/, temp/, .claude/tmp/) and flags files
  worth graduating (scripts, substantial docs, session-log-referenced) before
  they're lost to cleanup. Silent when scratch is empty.
- /save (Phase 4) and /scc (new step 2) run the check before sync.sh, pointing
  at .claude/TEMP_GRADUATION.md for the graduate-vs-delete decision.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -30,6 +30,7 @@ tmp-remediation/
 *.log
 *.bak
 .claude/tmp/
+tmp/
 
 # Live secrets / tokens — never commit
 .token