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-base.ts

@@ -260,6 +260,9 @@ export class AppDatabaseBase {
   incrementSessionUsage(sessionId: string, inputTokens: number, outputTokens: number, runId: string) { return this.tasks.incrementSessionUsage(sessionId, inputTokens, outputTokens, runId); }
   archiveSession(sessionId: string, reason: string, handoffMarkdown: string) { return this.tasks.archiveSession(sessionId, reason, handoffMarkdown); }
   createNewSession(input: { taskId: string; companyId: string; agentId: string; adapterType: string; sessionParamsJson: string | null; sessionDisplayId: string | null }) { return this.tasks.createNewSession(input); }
+  ensureAgentSession(input: { agentId: string; companyId: string; adapterType: string }) { return this.tasks.ensureAgentSession(input); }
+  setAgentSessionId(agentId: string, sessionId: string) { return this.tasks.setAgentSessionId(agentId, sessionId); }
+  updateTasksActiveSessionForAgent(agentId: string, sessionId: string) { return this.tasks.updateTasksActiveSessionForAgent(agentId, sessionId); }
   getRecentRunsForTask(taskId: string, limit: number) { return this.tasks.getRecentRunsForTask(taskId, limit); }
   getRecentActivityForTask(taskId: string, limit: number) { return this.tasks.getRecentActivityForTask(taskId, limit); }
   checkoutTask(taskId: string, runId: string) { return this.tasks.checkoutTask(taskId, runId); }