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  - tasks.session_json          (superseded by task_sessions.session_params_json)
  - tasks.session_display_id    (superseded by task_sessions.session_display_id)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/tasks-status.ts

@@ -107,7 +107,6 @@ export function buildTaskStatusMethods(
               t.parent_id as parentId, t.title, t.description,
               t.assignee_agent_id as assigneeAgentId, t.workspace_id as workspaceId,
               t.status, t.priority, t.active_run_id as activeRunId,
-              t.session_json as sessionJson, t.session_display_id as sessionDisplayId,
               t.cost_usd as costUsd,
               t.created_at as createdAt, t.updated_at as updatedAt
        from tasks t