feat(agent): Pi subagents via agent.spawn / agent.await

- agent_run gains depth / spawn_reason (0029); agent_run_event types
  subagent_spawned / subagent_event / subagent_completed
- agent.spawn (propose, medium, replay never): allowlist ⊆ parent, mode not
  above parent, budget carved from parent's remainder, idempotent per run+input;
  settings.pi.subagents {enabled, maxConcurrent, maxDepth} (subagentsEnabled kept as alias)
- child runs claim a concurrency slot under an advisory lock (waitForFact while
  queued); agent.await polls business tables without calling the model
- parent abort cascades to all descendants; parent runtime budget counts
  descendant usage; child events mirrored into the parent stream by parent seq
- authorize caps every child call by all ancestors' allowlists/kinds
- session page shows parent link, inherited info and the subagent tree
- e2e E11; runbook section on subagents

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/db/src/types.ts

@@ -47,7 +47,10 @@ export type AgentRunEventType =
   | "tool_call_rejected"
   | "compaction"
   | "run_completed"
-  | "run_failed";
+  | "run_failed"
+  | "subagent_spawned"
+  | "subagent_event"
+  | "subagent_completed";
 export type AgentRunMessageStatus = "pending" | "consumed";
 export type ApprovalStatus = "pending" | "approved" | "rejected" | "revoked" | "expired" | "consumed";
 export type ArtifactKind = "evidence" | "plan" | "proposal" | "report" | "snapshot" | "log";
@@ -344,6 +347,8 @@ export interface AgentRunTable {
   mode: Generated<AgentRunMode>;
   dedupe_key: string | null;
   upstream_id: string | null;
+  depth: Generated<number>;
+  spawn_reason: string | null;
 }
 
 export interface AgentRunEventTable {