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/snapshot.ts

@@ -83,7 +83,7 @@ export function buildSnapshotMethods(
         ...companyArgs,
       ),
       runs: readAll<RunRecord>(
-        `select id, company_id as companyId, task_id as taskId, agent_id as agentId, workspace_id as workspaceId, connector_id as connectorId, status, summary, error_message as errorMessage, exit_code as exitCode, signal, model, session_display_id as sessionDisplayId, cost_usd as costUsd, unattributed_cost as unattributedCost, started_at as startedAt, finished_at as finishedAt, log_path as logPath, created_at as createdAt, updated_at as updatedAt from runs${companyWhere} order by created_at desc limit ${companyId ? 2000 : 2000}`,
+        `select id, company_id as companyId, task_id as taskId, agent_id as agentId, workspace_id as workspaceId, connector_id as connectorId, status, summary, error_message as errorMessage, exit_code as exitCode, signal, model, cost_usd as costUsd, unattributed_cost as unattributedCost, started_at as startedAt, finished_at as finishedAt, log_path as logPath, created_at as createdAt, updated_at as updatedAt from runs${companyWhere} order by created_at desc limit ${companyId ? 2000 : 2000}`,
         ...companyArgs,
       ).map((record) => ({ ...record, unattributedCost: Boolean(record.unattributedCost) })) as RunRecord[],
       costs: readAll<CostRecord>(