refactor(session): scope pi sessions to agents instead of tasks

Sessions were keyed by (task, agent). Every task switch started a fresh
transcript — the agent lost everything it had learned on the previous
task. That breaks the product premise: a CEO should remember its past
decisions, an engineer should carry context between bugs.

Move pi's AgentMessage transcript to be agent-scoped. One session per
agent (per company). Tasks enter the ongoing conversation as user turns;
rotation/compaction boundaries follow the agent's long-running session.

Additive schema only — no row rewrites on the live production DB.

- Migration v34: `ALTER TABLE agents ADD COLUMN session_id TEXT` +
  `idx_agents_session_id`. `ensureOptionalColumns` mirror for fresh DBs.
- `AgentRecord.sessionId?: string | null` in the read model; included
  in every full-agent SELECT (entity-crud, snapshot, agents-hierarchy
  chain-of-command / direct-reports / heartbeat-eligible).
- `db.ensureAgentSession({ agentId, companyId, adapterType })` —
  idempotent: returns existing `agents.session_id` if set, else inserts
  a new `task_sessions` row with an empty `task_id` sentinel (agent-
  scoped, task-id is now metadata) and stores the id on the agent.
- `db.setAgentSessionId(agentId, sessionId)` — used by rotation.
- `db.updateTasksActiveSessionForAgent(agentId, sessionId)` — mirrors
  the agent's current session onto every task the agent owns, so
  existing per-task readers (`tasks.active_session_id`) still work.
- `turn-context.resolveSession`: pulls the agent's session via
  `ensureAgentSession` instead of `getActiveTaskSession(task.id)`,
  then points the current task at it so UI continues to resolve.
- `session-management-service.rotateSession`: new session carries
  `taskId:""`, then `setAgentSessionId` + `updateTasksActiveSessionForAgent`
  so every task the agent owns follows the rotation boundary.
- `session-entries.listActive`: `ORDER BY created_at, sequence` — the
  created_at timestamp is authoritative; sequence stays as same-ms
  tiebreaker. Forward-safety for future merge scenarios.

Existing agents' first post-upgrade run creates a fresh agent-scoped
session. Their pre-migration per-task transcripts remain in the DB as
historical records but are not loaded for new runs. This is a one-time
transcript reset, accepted over a risky lossy merge on the live DB.

Documented in a code comment on `ensureAgentSession`.

src/main/database/schema.ts

@@ -30,6 +30,8 @@ export function ensureOptionalColumns(db: DatabaseSync) {
   addColumnIfMissing("agents", "heartbeat_enabled", "alter table agents add column heartbeat_enabled integer not null default 0");
   addColumnIfMissing("agents", "heartbeat_interval_sec", "alter table agents add column heartbeat_interval_sec integer not null default 120");
   addColumnIfMissing("agents", "last_heartbeat_at", "alter table agents add column last_heartbeat_at text");
+  addColumnIfMissing("agents", "session_id", "alter table agents add column session_id text");
+  db.exec("create index if not exists idx_agents_session_id on agents(session_id)");
   addColumnIfMissing("companies", "auto_approve_hires", "alter table companies add column auto_approve_hires integer not null default 0");
   addColumnIfMissing("companies", "review_deliverables", "alter table companies add column review_deliverables integer not null default 0");
   addColumnIfMissing("companies", "execution_mode", "alter table companies add column execution_mode text not null default 'ai_ceo'");
@@ -501,6 +503,16 @@ export function runMigrations(
             );
             CREATE INDEX IF NOT EXISTS idx_credentials_status ON credentials(status)`,
     },
+    {
+      // Per-agent session scope. Agents carry a single active session across
+      // tasks — switching tasks becomes a "now work on X" user turn on the
+      // agent's ongoing transcript. Additive-only: existing task_sessions
+      // rows remain as historical records; new runs use the agent-scoped
+      // session created lazily on first post-upgrade run.
+      version: 34, name: "add_agents_session_id",
+      sql: `ALTER TABLE agents ADD COLUMN session_id TEXT;
+            CREATE INDEX IF NOT EXISTS idx_agents_session_id ON agents(session_id)`,
+    },
   ];
 
   for (const migration of migrations) {