fix: prevent tmux detach from stalling Happy CLI

[dzlobin]

Summary

• Wraps Ink's stdout in a non-blocking proxy that drops writes when PTY backpressure is detected (e.g. tmux detach), preventing the Node.js event loop from blocking
• Applied consistently across Claude, Codex, and Gemini agent launchers
• Includes 12 unit tests covering normal writes, backpressure detection, drain recovery, and listener accumulation prevention

Root Cause

When tmux detaches, the PTY buffer (~4KB on macOS) fills with no reader. process.stdout.write() then blocks synchronously, freezing the entire Node.js event loop and stalling all Happy instances in that tmux session.

Fix

A Proxy wraps process.stdout and intercepts write() calls. When writableNeedDrain is true or write() returns false, subsequent writes are silently dropped (with callbacks still invoked) until a drain event fires. This is passed to Ink's render() via the stdout option.

Closes #533

Test plan

• 12 unit tests passing (vitest)
• Manual testing: run happy in tmux, detach, verify no stall, reattach and confirm UI resumes
• Test with multiple instances in same tmux session

🤖 Generated with Claude Code