Add Phase 3 CLI tools: exo client and exod server binaries

- exod: gRPC daemon with auto-migration, graceful shutdown (SIGINT/SIGTERM),
  configurable listen address and data directory via flags
- exo: CLI client with import (YAML artifacts), tag add/list, cat add/list,
  search by tag commands; connects via EXO_ADDR env var
- Add bin/ to .gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,2 +1,3 @@
 /ark
-*.db
+*.db
+bin/