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.

tests/unit/database.test.ts

@@ -296,7 +296,6 @@ describe("AppDatabase", () => {
       exitCode: 0,
       signal: null,
       model: "gpt-5",
-      sessionDisplayId: "session-budget",
       unattributedCost: false,
     });
 
@@ -515,7 +514,6 @@ describe("AppDatabase", () => {
       exitCode: 0,
       signal: null,
       model: null,
-      sessionDisplayId: null,
       unattributedCost: true,
     });