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/spawn-claude-md.ts

@@ -0,0 +1,154 @@
+/**
+ * @fileoverview Agent CLAUDE.md Generator for spawn1337 protocol.
+ *
+ * Generates a comprehensive CLAUDE.md for each spawned agent that tells it:
+ * - What its task is
+ * - How to communicate progress
+ * - How to signal completion
+ * - What constraints it has
+ * - How to read/write messages
+ *
+ * @module spawn-claude-md
+ */
+
+import type { SpawnTask } from './spawn-types.js';
+
+/**
+ * Generate a CLAUDE.md file for a spawned agent.
+ *
+ * This CLAUDE.md gives the agent full context about:
+ * - Its identity and task
+ * - Communication protocol (progress, messages, result)
+ * - Resource constraints (timeout, tokens, cost)
+ * - Working directory and available context files
+ *
+ * @param task - The full parsed task specification
+ * @param commsDir - Absolute path to the communication directory
+ * @param agentWorkingDir - Absolute path to the agent's working directory
+ * @returns The generated CLAUDE.md content
+ */
+export function generateAgentClaudeMd(task: SpawnTask, commsDir: string, agentWorkingDir: string): string {
+  const spec = task.spec;
+
+  const constraintLines: string[] = [];
+  constraintLines.push(`- Timeout: ${spec.timeoutMinutes} minutes`);
+  if (spec.maxTokens) constraintLines.push(`- Token budget: ${spec.maxTokens.toLocaleString()} tokens`);
+  if (spec.maxCost) constraintLines.push(`- Cost budget: $${spec.maxCost.toFixed(2)}`);
+  if (!spec.canModifyParentFiles) {
+    constraintLines.push('- DO NOT modify files outside your workspace');
+  } else {
+    constraintLines.push('- You MAY modify files in the parent project directory');
+  }
+  constraintLines.push(`- Output format: ${spec.outputFormat}`);
+
+  const contextSection = spec.contextFiles && spec.contextFiles.length > 0
+    ? `\nContext files available in workspace:\n${spec.contextFiles.map(f => `- ${f}`).join('\n')}`
+    : '';
+
+  const progressSection = spec.progressIntervalSeconds > 0
+    ? `### Progress Reporting
+
+Update \`${commsDir}/progress.json\` every ~${spec.progressIntervalSeconds} seconds with your current status:
+
+\`\`\`json
+{
+  "phase": "current phase description",
+  "percentComplete": 45,
+  "currentAction": "What you are doing right now",
+  "subtasks": [
+    {"description": "Subtask 1", "status": "completed"},
+    {"description": "Subtask 2", "status": "in_progress"}
+  ],
+  "filesModified": ["file1.ts", "file2.ts"],
+  "tokensUsed": 0,
+  "costSoFar": 0,
+  "updatedAt": ${Date.now()}
+}
+\`\`\``
+    : '### Progress Reporting\n\nProgress reporting is disabled for this task.';
+
+  return `# Agent: ${spec.name}
+
+## Your Identity
+
+You are an autonomous agent (ID: \`${spec.agentId}\`) spawned by a parent Claude session.
+You are running in your own screen session with full Claude Code capabilities.
+Type: ${spec.type} | Priority: ${spec.priority} | Depth: ${task.depth}
+
+## Task
+
+${task.instructions}
+
+## Success Criteria
+
+${spec.successCriteria || 'Complete the task as described above.'}
+
+## Communication Protocol
+
+${progressSection}
+
+### Check for Messages
+
+Periodically check \`${commsDir}/messages/\` for instructions from the parent.
+Files are named \`NNN-parent.md\` (from parent) or \`NNN-agent.md\` (from you).
+Read any new \`*-parent.md\` files for additional instructions or clarifications.
+
+To send a message back to the parent, create a file like:
+\`${commsDir}/messages/002-agent.md\`
+
+### Write Result
+
+When complete, write your final result to \`${commsDir}/result.md\` with YAML frontmatter:
+
+\`\`\`markdown
+---
+status: completed
+summary: "Brief 1-3 sentence summary of what you accomplished"
+filesChanged:
+  - path: relative/path/to/file.ts
+    action: modified
+    summary: "What was changed"
+---
+
+## Detailed Output
+
+Your full output, analysis, or report here.
+\`\`\`
+
+Valid status values: \`completed\`, \`failed\`
+
+### Signal Completion
+
+After writing result.md, output this EXACT phrase to signal you are done:
+
+<promise>${spec.completionPhrase}</promise>
+
+**IMPORTANT**: Only output the completion phrase AFTER you have written result.md.
+The completion phrase triggers the orchestrator to read your result and clean up.
+
+## Constraints
+
+${constraintLines.join('\n')}
+
+## Working Directory
+
+Your workspace is: \`${agentWorkingDir}\`
+${contextSection}
+
+## Important Notes
+
+- Work autonomously - do not ask for user input
+- Focus exclusively on the task described above
+- If you encounter errors, document them in result.md with status: failed
+- Do not modify this CLAUDE.md file
+- Stay within your resource constraints
+`;
+}
+
+/**
+ * Build the initial prompt injected into the agent session via writeViaScreen().
+ * Intentionally brief - all detail is in the CLAUDE.md.
+ */
+export function buildInitialPrompt(task: SpawnTask): string {
+  return `Read your CLAUDE.md file for complete task instructions, communication protocol, and constraints. Begin working on the task immediately. Report progress to spawn-comms/progress.json periodically. When complete, write your result to spawn-comms/result.md and then output your completion phrase: <promise>${task.spec.completionPhrase}</promise>`;
+}