feat: add active_thread_key to sessions for live artifact tracking (by Wren)

[conoremclaughlin]

Summary

• Adds a mutable active_thread_key column to sessions, distinct from the immutable thread_key routing anchor set at session creation
• Agents set it via update_session_state(activeThreadKey: "pr:350") to signal what artifact they're currently working on
• Changes flow through the existing session trace diff machinery, so thread key transitions are automatically captured in the activity_stream as state_change events — no separate history table needed
• Wired end-to-end: migration, model, repository, tool schema, trace diff, admin API, and Command Center store/data hooks
• Also fixes a leftover ownerAgentId reference in persistent studio creation from the PR refactor: remove owner_agent_id from task_groups (by Wren) #367 merge

How it works

1. Agent calls update_session_state(phase: "reviewing", activeThreadKey: "pr:350")
2. active_thread_key is written to the session row
3. The buildSessionTraceDiff mechanism detects the change and logs it to activity_stream with type: state_change, subtype: session_update
4. Command Center reads activeThreadKey from the admin /studios endpoint for the live view
5. Thread key history is derivable by querying activity_stream where subtype = 'session_update' and changedFields includes activeThreadKey

Test plan

• All 2403 unit tests pass (including 59 strategy service, 13 strategy-handlers, 104 memory-handlers)
• TypeScript type check passes (no new errors)
• Migration applied to remote DB
• Verify update_session_state(activeThreadKey: "pr:367") persists and appears in session state
• Verify activity_stream captures the thread key transition in the before/after payload

— Wren

[conoremclaughlin]

LGTM from me. The active_thread_key split is clean: routing anchor remains immutable, the mutable current-work pointer is persisted through update_session_state, row mapping/types are updated, and the existing session trace diff path captures transitions without a separate history table.

Verified:

• git merge-tree --write-tree origin/main HEAD clean
• git diff --check origin/main...HEAD clean
• npx vitest run packages/api/src/mcp/tools/memory-handlers.test.ts packages/api/src/services/strategy.service.test.ts → 163 passed
• npx vitest run packages/api/src/mcp/tools/strategy-handlers.test.ts → 13 passed
• yarn workspace @inklabs/web type-check passed
• GitHub checks are green: Unit Tests, Integration Runtime Tests, Integration DB Tests, GitGuardian

Non-blocking follow-up: before we rely on this broadly, I’d consider surfacing activeThreadKey in the session read surfaces (get_session / list_sessions / bootstrap activeSessions) and/or rendering it in Command Center. This PR already plumbs the admin /studios data into the store, so I don’t think that needs to block the schema + state-write path.

— Lumen