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/migrations/v43-v43.ts

@@ -0,0 +1,143 @@
+import log from "electron-log/main.js";
+import type { Migration, MigrationDeps } from "./types";
+import { columnExists, tableExists } from "./types";
+
+/**
+ * Drop the Pi Mono-dead session carry-forward columns.
+ *
+ * The `task_sessions.session_params_json` / `task_sessions.session_display_id`
+ * columns and the matching `runs.session_display_id` column were meaningful
+ * in the CLI-adapter era: Gemini's resume id and provider-specific session
+ * params rode the run-finished message back into 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 writing literal `null` on every
+ * finish and `rotateSession` has been carrying `null` forward.
+ *
+ * Three columns of unused storage plus a hot path that writes them every
+ * run. Drop them.
+ *
+ * The migration mirrors v41's "copy to new table, swap, reindex" shape —
+ * SQLite still has no multi-column `DROP COLUMN`, and the task_sessions
+ * index topology plus the runs secondary indexes need to be restored
+ * identically. `PRAGMA foreign_keys` is disabled across the swap window
+ * so child rows are not cascade-deleted mid-rebuild.
+ */
+export function createMigrationsV43V43(deps: MigrationDeps): Migration[] {
+  const { db } = deps;
+  return [
+    {
+      version: 43,
+      name: "drop_dead_pi_mono_session_columns",
+      fn: () => {
+        const sessionsHasDeadCols =
+          tableExists(db, "task_sessions") &&
+          (columnExists(db, "task_sessions", "session_params_json") ||
+            columnExists(db, "task_sessions", "session_display_id"));
+        const runsHasDeadCol =
+          tableExists(db, "runs") && columnExists(db, "runs", "session_display_id");
+
+        if (!sessionsHasDeadCols && !runsHasDeadCol) {
+          log.info("[migration v43] dead columns already absent; skipping");
+          return;
+        }
+
+        db.exec("PRAGMA foreign_keys = OFF");
+        try {
+          if (sessionsHasDeadCols) {
+            db.exec(`
+              CREATE TABLE task_sessions_new (
+                id TEXT PRIMARY KEY,
+                company_id TEXT NOT NULL,
+                agent_id TEXT NOT NULL,
+                adapter_type TEXT NOT NULL,
+                last_run_id TEXT,
+                last_error TEXT,
+                run_count INTEGER NOT NULL DEFAULT 0,
+                total_input_tokens INTEGER NOT NULL DEFAULT 0,
+                total_output_tokens INTEGER NOT NULL DEFAULT 0,
+                rotated_at TEXT,
+                rotation_reason TEXT,
+                handoff_markdown TEXT,
+                created_at TEXT NOT NULL,
+                updated_at TEXT NOT NULL
+              );
+              INSERT INTO task_sessions_new (
+                id, company_id, agent_id, adapter_type, last_run_id, last_error,
+                run_count, total_input_tokens, total_output_tokens, rotated_at,
+                rotation_reason, handoff_markdown, created_at, updated_at
+              )
+              SELECT
+                id, company_id, agent_id, adapter_type, last_run_id, last_error,
+                run_count, total_input_tokens, total_output_tokens, rotated_at,
+                rotation_reason, handoff_markdown, created_at, updated_at
+              FROM task_sessions;
+              DROP TABLE task_sessions;
+              ALTER TABLE task_sessions_new RENAME TO task_sessions;
+              CREATE INDEX IF NOT EXISTS idx_task_sessions_company_agent
+                ON task_sessions(company_id, agent_id, updated_at);
+              CREATE INDEX IF NOT EXISTS idx_task_sessions_lookup
+                ON task_sessions(company_id, agent_id, adapter_type);
+            `);
+            log.info("[migration v43] rebuilt task_sessions without session_params_json/session_display_id");
+          }
+
+          if (runsHasDeadCol) {
+            // v16-v25 added `context_snapshot_json`; it is unreferenced by
+            // production code today but the column exists on real databases
+            // and carries no policy against preservation. Keep it so this
+            // migration's blast radius stays exactly one column.
+            db.exec(`
+              CREATE TABLE runs_new (
+                id TEXT PRIMARY KEY,
+                company_id TEXT NOT NULL,
+                task_id TEXT,
+                agent_id TEXT NOT NULL,
+                workspace_id TEXT,
+                connector_id TEXT NOT NULL,
+                status TEXT NOT NULL,
+                summary TEXT NOT NULL,
+                error_message TEXT,
+                exit_code INTEGER,
+                signal TEXT,
+                model TEXT,
+                cost_usd REAL,
+                unattributed_cost INTEGER NOT NULL DEFAULT 0,
+                context_snapshot_json TEXT,
+                actor_type TEXT,
+                actor_id TEXT,
+                started_at TEXT,
+                finished_at TEXT,
+                log_path TEXT,
+                created_at TEXT NOT NULL,
+                updated_at TEXT NOT NULL
+              );
+              INSERT INTO runs_new (
+                id, company_id, task_id, agent_id, workspace_id, connector_id,
+                status, summary, error_message, exit_code, signal, model,
+                cost_usd, unattributed_cost, context_snapshot_json, actor_type,
+                actor_id, started_at, finished_at, log_path, created_at, updated_at
+              )
+              SELECT
+                id, company_id, task_id, agent_id, workspace_id, connector_id,
+                status, summary, error_message, exit_code, signal, model,
+                cost_usd, unattributed_cost, context_snapshot_json, actor_type,
+                actor_id, started_at, finished_at, log_path, created_at, updated_at
+              FROM runs;
+              DROP TABLE runs;
+              ALTER TABLE runs_new RENAME TO runs;
+              CREATE INDEX IF NOT EXISTS idx_runs_company ON runs(company_id);
+              CREATE INDEX IF NOT EXISTS idx_runs_task ON runs(task_id);
+              CREATE INDEX IF NOT EXISTS idx_runs_agent ON runs(agent_id);
+              CREATE INDEX IF NOT EXISTS idx_runs_company_status ON runs(company_id, status);
+              CREATE INDEX IF NOT EXISTS idx_runs_company_created_at ON runs(company_id, created_at DESC);
+              CREATE INDEX IF NOT EXISTS idx_runs_agent_created_at ON runs(agent_id, created_at DESC);
+            `);
+            log.info("[migration v43] rebuilt runs without session_display_id");
+          }
+        } finally {
+          db.exec("PRAGMA foreign_keys = ON");
+        }
+      },
+    },
+  ];
+}