fix(deletions): track deletions.jsonl in git for cross-clone propagation

Fixes bd-44e: Remove deletions.jsonl from .gitignore.

The deletions manifest MUST be tracked in git for cross-clone deletion
propagation to work (bd-imj). Previously it was incorrectly gitignored
with a "local-only" comment that contradicted the design.

Changes:
- Remove .beads/deletions.jsonl from .gitignore
- Add comment explaining why it is tracked
- Track existing deletions.jsonl file

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

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

.gitignore

@@ -58,8 +58,8 @@ Thumbs.db
 .beads/beads.base.meta.json
 .beads/beads.left.meta.json
 
-# Cleanup deletion tracking (local-only)
-.beads/deletions.jsonl
+# Note: .beads/deletions.jsonl is intentionally NOT ignored
+# It must be tracked in git for cross-clone deletion propagation (bd-imj)
 
 # Git merge driver temp files (created during conflicts with numbered extensions)
 .beads/*.json[0-9]