chore(git): tighten .gitignore and untrack machine-specific files

- Add *.db-shm and *.db-wal to cover SQLite WAL mode files (*.db alone
  was not sufficient)
- Replace blanket .claude/ ignore with .claude/ + !.claude/rules/ so
  project-level Claude Code rules remain tracked while local settings
  (settings.local.json, projects/, etc.) stay ignored

Machine-specific files removed from tracking in the previous refactor
commit: .claude/settings.local.json, local.db-shm, local.db-wal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -23,7 +23,16 @@ vite.config.js.timestamp-*
 vite.config.ts.timestamp-*
 # SQLite
 *.db
+*.db-shm
+*.db-wal
 
 # Pipeline artifacts
 prompts/
 docs/docs_cache_state.yaml
+
+# Tmp
+.tmp/
+
+# Claude Code — ignore local/machine-specific settings, keep project rules
+.claude/
+!.claude/rules/