Add Phase 1 MVP roadmap for Ghostty WASM terminal (#3)

- Add comprehensive 8-task breakdown for terminal emulator
- Include project vision: leverage Ghostty WASM parser, build UI in TypeScript
- Define xterm.js-compatible API goals
- Configure for Bun + bun:test (matching cmux environment)
- Target Chrome 90+ for Phase 1
- Estimate 3 weeks with parallel agent work
- Include code scaffolds, test templates, and dependencies graph

Author: sreya