refactor(session): drop dead task-scoped session columns (v41)

Sessions have been agent-scoped since v34, but the schema kept four
dead task-scoped columns that every session code path had to pretend
were meaningful:

  - tasks.active_session_id     (mirrored agents.session_id)
  - tasks.session_json          (superseded by task_sessions.session_params_json)
  - tasks.session_display_id    (superseded by task_sessions.session_display_id)
  - task_sessions.task_id       (always empty-string on new rows)

v41 drops all four via SQLite table rebuild — the only safe path when
multiple columns need to go in one step and indexes / FK directions
need to be preserved. `PRAGMA foreign_keys = OFF` spans the swap
window so cascade-delete does not fire mid-rebuild. The migration is
idempotent and also backfills the optional task columns
(task_type / requires_user_review / retry_count / last_failure_at /
started_at / completed_at / cancelled_at / checkout_run_id /
execution_locked_at) before rebuilding so it works whether or not
`ensureOptionalColumns` has run yet on this install.

Along with the schema change, every code path that read or wrote
those columns has been rewired to the agent-scoped equivalents:

- task-sessions-crud.ts: deleted updateTaskSession,
  getTaskSession, getActiveTaskSession, updateTasksActiveSessionForAgent;
  added updateSessionParams(sessionId, ...) for the worker's
  finalizer. ensureAgentSession and createNewSession no longer touch
  task_id.
- database-base.ts: facade drops the now-unused pass-throughs.
- snapshot.ts / task-queries.ts / tasks-status.ts / entity-crud.ts:
  SELECT projections no longer pull session_json / session_display_id
  / active_session_id from tasks.
- tasks-crud.ts (TaskRow) and shared/types/records-core.ts
  (TaskRecord): sessionJson / sessionDisplayId fields removed.
- TaskSessionRecord: taskId field removed.
- turn-context.ts::resolveSession: no more
  updateTasksActiveSessionForAgent mirror; sessionParams read directly
  from the current session via getSessionById.
- run-finished-helpers.ts::finalizeRun: stamps sessionParams via
  updateSessionParams on the agent's current session, resolved through
  agents.session_id.
- session-management-service.ts: rotateSession stops calling
  updateTasksActiveSessionForAgent.

Test suite updated to use the new session-id API. Existing tests used
`getActiveTaskSession(taskId)` and `createNewSession({ taskId, ...})`
as probes; they now use `getSessionById(sessionId)` and the taskId-less
createNewSession input.

Tests: 603/603 green. Typecheck clean. Migration tested on fresh DB
(runs v1→v41 cleanly) and idempotent re-apply (detects absent dead
columns, skips).

src/main/database-base.ts

@@ -339,16 +339,13 @@ export class AppDatabaseBase {
   saveTask(input: { id?: string | null; companyId: string; projectId?: string | null; goalId?: string | null; parentId?: string | null; title: string; description: string; assigneeAgentId?: string | null; workspaceId?: string | null; priority: string; status: string }) { return this.tasks.saveTask(input); }
   getTask(id: string) { return this.entityCrud.getTask(id); }
   deleteTask(id: string, companyId: string) { return this.tasks.deleteTask(id, companyId); }
-  updateTaskSession(taskId: string, sessionJson: Record<string, unknown> | null, sessionDisplayId: string | null) { return this.tasks.updateTaskSession(taskId, sessionJson, sessionDisplayId); }
-  getTaskSession(taskId: string) { return this.tasks.getTaskSession(taskId); }
-  getActiveTaskSession(taskId: string) { return this.tasks.getActiveTaskSession(taskId); }
+  updateSessionParams(sessionId: string, sessionParams: Record<string, unknown> | null, sessionDisplayId: string | null) { return this.tasks.updateSessionParams(sessionId, sessionParams, sessionDisplayId); }
   getSessionById(sessionId: string) { return this.tasks.getSessionById(sessionId); }
   incrementSessionUsage(sessionId: string, inputTokens: number, outputTokens: number, runId: string) { return this.tasks.incrementSessionUsage(sessionId, inputTokens, outputTokens, runId); }
   archiveSession(sessionId: string, reason: string, handoffMarkdown: string) { return this.tasks.archiveSession(sessionId, reason, handoffMarkdown); }
-  createNewSession(input: { taskId: string; companyId: string; agentId: string; adapterType: string; sessionParamsJson: string | null; sessionDisplayId: string | null }) { return this.tasks.createNewSession(input); }
+  createNewSession(input: { companyId: string; agentId: string; adapterType: string; sessionParamsJson: string | null; sessionDisplayId: string | null }) { return this.tasks.createNewSession(input); }
   ensureAgentSession(input: { agentId: string; companyId: string; adapterType: string }) { return this.tasks.ensureAgentSession(input); }
   setAgentSessionId(agentId: string, sessionId: string) { return this.tasks.setAgentSessionId(agentId, sessionId); }
-  updateTasksActiveSessionForAgent(agentId: string, sessionId: string) { return this.tasks.updateTasksActiveSessionForAgent(agentId, sessionId); }
   getRecentRunsForTask(taskId: string, limit: number) { return this.tasks.getRecentRunsForTask(taskId, limit); }
   getRecentActivityForTask(taskId: string, limit: number) { return this.tasks.getRecentActivityForTask(taskId, limit); }
   checkoutTask(taskId: string, runId: string) { return this.tasks.checkoutTask(taskId, runId); }