[ENHANCEMENT] History Storage #1313

Summary

Task history persistence uses api_conversation_history.json and ui_messages.json.
Both files are full JSON arrays. On each write, the whole file is read, parsed, an object appended, stringified, and saved again. This is unsuitable for logging.

Proposal (phased)

P0 – stop the bleeding

• Implemented: fix(task-history): atomic per-task merge and drop shared index file (#1231) #1261

P1 – optional workspace indices

• Workspace-scoped indexes with only the task-ids in an array (helping to only read relevant tasks from the directory, since the global index is now gone)
• Narrow writes: only on task creation or deletion, one single entry is added/removed
• Using save writes with updated return value: refactor(safeWriteJson): return SafeWriteResult with dataWritten for cache synchronization #1304

P2 – durable design for #355

• Append-only event log (JSONL or line protocol) or SQLite (WAL) task catalog.
• Repository interface shared by GUI, CLI, and repair tools.