feat: implement spawn1337 autonomous agent protocol

Add full lifecycle management for spawning autonomous Claude sessions as
screen-based agents. Agents communicate via filesystem message bus, signal
completion via RalphTracker <promise> mechanism, and enforce resource budgets
(tokens, cost, timeout, depth limits).

New files:
- spawn-types.ts: Types, YAML parser, factory functions, serialization
- spawn-detector.ts: Terminal pattern detection for spawn1337 tags
- spawn-orchestrator.ts: Agent lifecycle (spawn, monitor, queue, cleanup)
- spawn-claude-md.ts: CLAUDE.md generator for agent sessions

Modified:
- session.ts: SpawnDetector integration, parent/child tracking
- server.ts: Orchestrator wiring, 11 API endpoints, SSE events
- types.ts: Re-exports, SessionState additions

Tests: 80 new tests across 3 test files (all passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.ts

@@ -82,6 +82,10 @@ export interface SessionState {
   ralphEnabled?: boolean;
   /** Ralph completion phrase (if set) */
   ralphCompletionPhrase?: string;
+  /** Parent agent ID if this session is a spawned agent */
+  parentAgentId?: string;
+  /** Child agent IDs spawned by this session */
+  childAgentIds?: string[];
 }
 
 // ========== Task Types ==========
@@ -722,3 +726,21 @@ export function getErrorMessage(error: unknown): string {
   }
   return 'An unknown error occurred';
 }
+
+// ========== Spawn1337 Protocol Re-exports ==========
+
+export type {
+  SpawnPriority,
+  SpawnResultDelivery,
+  SpawnStatus,
+  SpawnTaskSpec,
+  SpawnTask,
+  AgentProgress,
+  SpawnResult,
+  SpawnMessage,
+  AgentStatusReport,
+  SpawnTrackerState,
+  SpawnOrchestratorConfig,
+  AgentContext,
+  SpawnPersistedState,
+} from './spawn-types.js';