* Update default values for metadata and messages columns in thoughts and chat_histories tables
* Change tags column type to text[] with default in learnings, plans, agent_skills, and agent_guardrails tables
* Adjust index creation for tags columns in learnings and plans tables
All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.
