feat(deletions): complete deletions manifest epic with integration tests

Completes the deletion propagation epic (bd-imj) with all 9 subtasks:
- Cross-clone deletion propagation via deletions.jsonl
- bd deleted command for audit trail
- Auto-compact during sync (opt-in)
- Git history fallback with timeout and regex escaping
- JSON output for pruning results
- Integration tests for deletion scenarios
- Documentation in AGENTS.md, README.md, and docs/DELETIONS.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -168,6 +168,7 @@ echo ".beads/beads.jsonl merge=beads" >> .gitattributes
 **Should be committed to git:**
 - `.gitattributes` - Configures git merge driver for intelligent JSONL merging (critical for team collaboration)
 - `.beads/beads.jsonl` - Issue data in JSONL format (source of truth, synced via git)
+- `.beads/deletions.jsonl` - Deletion manifest for cross-clone propagation (tracks deleted issues)
 - `.beads/config.yaml` - Repository configuration template
 - `.beads/README.md` - Documentation about beads for repository visitors
 - `.beads/metadata.json` - Database metadata