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/orchestration/turn-context.ts

@@ -127,11 +127,24 @@ function resolveEnabledConnectorModels(
 
 function resolveSession(
   db: AppDatabase,
+  agent: RunDispatchParams["agent"],
   task: RunDispatchParams["task"],
+  companyId: string,
   connectorId: string,
   sessionHandoff: string | null,
   deps: Pick<TurnContextDeps, "evaluatePreRunSessionRotation" | "logger">,
 ) {
+  // Per-agent session: one ongoing transcript per agent; switching tasks
+  // becomes a "now work on X" user turn rather than a fresh session.
+  const agentSessionId = db.ensureAgentSession({
+    agentId: agent.id,
+    companyId,
+    adapterType: connectorId,
+  });
+  // Mirror the agent's active session onto the task pointer so UI paths
+  // that read per-task active_session_id keep working.
+  db.updateTasksActiveSessionForAgent(agent.id, agentSessionId);
+
   const activeSession = db.getActiveTaskSession(task.id);
   let taskSession: {
     id: string | null;
@@ -145,7 +158,7 @@ function resolveSession(
           : null,
         sessionDisplayId: activeSession.sessionDisplayId,
       }
-    : { id: null, ...db.getTaskSession(task.id) };
+    : { id: agentSessionId, sessionParams: null, sessionDisplayId: null };
 
   let finalHandoff = sessionHandoff;
   if (deps.evaluatePreRunSessionRotation && activeSession) {
@@ -155,7 +168,7 @@ function resolveSession(
       if (!finalHandoff && rotationResult.handoff) {
         finalHandoff = rotationResult.handoff;
       }
-      deps.logger.info(`[session] Pre-run compaction for task ${task.id}: session rotated before execution`);
+      deps.logger.info(`[session] Pre-run compaction for agent ${agent.id}: session rotated before execution`);
     }
   }
 
@@ -260,7 +273,7 @@ export function buildRunPayload(params: RunDispatchParams, deps: TurnContextDeps
   const reviewTasks = db.listReviewTasksForManager(agent.id, companyId);
   const recentComments = assembleRecentComments(assignedTasks, reviewTasks, snapshot);
   const initialHandoff = resolveSessionHandoff(task.id, deps);
-  const { taskSession, sessionHandoff } = resolveSession(db, task, connector.id, initialHandoff, deps);
+  const { taskSession, sessionHandoff } = resolveSession(db, agent, task, companyId, connector.id, initialHandoff, deps);
   const skills = resolveSkills(companyId, agent.id, connector.id, agent.name, deps);
 
   const systemPrompt = apiUrl ? buildTaskPrompt({