refactor(session): nuke dead sessionParams / sessionDisplayId pipeline (v43)

These columns and the messages that plumbed them existed for the CLI-
adapter era — Gemini's resume id and provider-specific session params
rode the run-finished envelope back onto the session row so the next
turn could resume. Pi Mono dispatches every provider through pi-ai and
emits neither value. The worker has been posting literal null on every
finish, finalizeRun has been writing null into task_sessions, and
rotateSession has been carrying null forward.

Remove the wire, the writer, the carry-forward, and the columns:

  - v43 migration rebuilds task_sessions without session_params_json /
    session_display_id, and runs without session_display_id. Follows
    the idempotent "create new, copy, swap, reindex" pattern used by
    v41. context_snapshot_json is preserved (also unreferenced today
    but outside this cleanup's scope).
  - RunJob / EnqueueRunPayload / ChildMessage / worker-types zod
    schema no longer carry the fields. finishRun() / createNewSession()
    signatures tighten in lockstep.
  - rotateSession starts the new session with fresh counters and no
    carry-forward.
  - turn-context resolveSession returns just the session id.
  - TaskSessionRecord / RunRecord lose the dead fields.

498/498 unit tests pass. typecheck + build clean.

src/main/database/schema.ts

@@ -7,6 +7,7 @@ import { createMigrationsV38V39 } from "./migrations/v38-v39";
 import { createMigrationsV40V40 } from "./migrations/v40-v40";
 import { createMigrationsV41V41 } from "./migrations/v41-v41";
 import { createMigrationsV42V42 } from "./migrations/v42-v42";
+import { createMigrationsV43V43 } from "./migrations/v43-v43";
 import {
   columnExists,
   nowIso,
@@ -180,6 +181,7 @@ export function runMigrations(
     ...createMigrationsV40V40(deps),
     ...createMigrationsV41V41(deps),
     ...createMigrationsV42V42(deps),
+    ...createMigrationsV43V43(deps),
   ];
 
   for (const migration of migrations) {