Add Zod runtime validation for agent output and IPC payloads

[gregpriday]

Summary

Implements runtime validation using Zod schemas to protect against malformed data at system boundaries (agent events, IPC payloads, and AI responses).

Closes #80

Changes Made

• Added electron/schemas/ module with comprehensive Zod schemas for:
  • Agent events (agent.ts): spawned, state-changed, output, completed, failed, killed
  • IPC payloads (ipc.ts): terminal operations, dev server controls, CopyTree integration
  • External data (external.ts): package.json, AI responses, git output
• Implemented validation in PtyManager for all agent event emissions
• Added validation to IPC handlers for terminal spawn/resize, dev server operations, and CopyTree generation/injection
• Enhanced AI response parsing with Zod validation and safe fallbacks
• Fixed agent:output bypass that could emit invalid payloads to transcript consumers
• Filtered undefined environment variables before PTY spawn to prevent node-pty errors
• Enforced non-empty strings in AgentOutputSchema and AgentFailedSchema
• Added safe color defaults for invalid AI-generated project identities
• Re-validated AI response fallbacks to prevent constraint bypass

Test Plan

• ✅ TypeScript type checking passes
• ✅ ESLint checks pass
• ✅ Prettier formatting applied
• ✅ All schemas compile correctly
• ✅ Validation errors are properly logged