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/worker/handlers/run-finished-helpers.ts

@@ -47,7 +47,6 @@ export function finalizeRun(
       exitCode: event.exitCode,
       signal: event.signal,
       model: event.model,
-      sessionDisplayId: event.sessionDisplayId,
       unattributedCost: event.unattributedCost,
       inputTokens: event.inputTokens,
       outputTokens: event.outputTokens,
@@ -66,21 +65,6 @@ export function finalizeRun(
     }
   }
 
-  // Stamp pi-ai's transient per-turn state (session resume params,
-  // display id) onto the agent's current session row. Sessions are
-  // agent-scoped, so the session id comes from agents.session_id —
-  // not from a task pointer (that indirection was dropped in v41).
-  try {
-    const agent = db.getAgent(run.agentId);
-    if (agent.sessionId) {
-      db.updateSessionParams(agent.sessionId, event.sessionParams, event.sessionDisplayId ?? null);
-    }
-  } catch (error) {
-    logger.warn(
-      `[finalizeRun] updateSessionParams failed for run ${event.runId}: ${error instanceof Error ? error.message : String(error)}`,
-    );
-  }
-
   leaseRenewalMap.delete(event.runId);
   const runOutcome = db.finishRun(event);
   if (run.workspaceId) {