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/snapshot.ts

@@ -63,7 +63,7 @@ export function buildSnapshotMethods(
         ...companyArgs,
       ).map((record) => ({ ...record, isPrimary: toBoolean(record.isPrimary) })) as WorkspaceRecord[],
       agents: readAll<SqliteRow>(
-        `select id, company_id as companyId, name, role, title, department, status, reports_to as reportsTo, connector_id as connectorId, workspace_id as workspaceId, model, capabilities, role_level as roleLevel, budget_monthly_usd as budgetMonthlyUsd, spent_monthly_usd as spentMonthlyUsd, metadata_json as metadataJson, heartbeat_enabled as heartbeatEnabled, heartbeat_interval_sec as heartbeatIntervalSec, last_heartbeat_at as lastHeartbeatAt, created_at as createdAt, updated_at as updatedAt from agents${companyWhere} order by created_at asc`,
+        `select id, company_id as companyId, name, role, title, department, status, reports_to as reportsTo, connector_id as connectorId, workspace_id as workspaceId, model, capabilities, role_level as roleLevel, budget_monthly_usd as budgetMonthlyUsd, spent_monthly_usd as spentMonthlyUsd, metadata_json as metadataJson, heartbeat_enabled as heartbeatEnabled, heartbeat_interval_sec as heartbeatIntervalSec, last_heartbeat_at as lastHeartbeatAt, session_id as sessionId, created_at as createdAt, updated_at as updatedAt from agents${companyWhere} order by created_at asc`,
         ...companyArgs,
       ).map((record: SqliteRow) => ({ ...record, heartbeatEnabled: toBoolean(record.heartbeatEnabled) })) as AgentRecord[],
       goals: readAll<GoalRecord>(