fix: terminal PTY write/resize errors silently swallowed

[xiaolai]

Closes #698

Summary

• Replace empty .catch(() => {}) blocks in all 7 PTY IPC calls with proper error logging
• pty_write and pty_resize failures use ptyWarn (persisted to log file in production) — these indicate data loss or geometry mismatch
• pty_kill, pty_pause, pty_resume, pty_close failures use terminalLog (dev-only debug) — fire-and-forget semantics are acceptable
• Added ptyWarn logger to src/utils/debug.ts
• Non-Error rejection values handled via instanceof guard

Test plan

• 6 new tests verifying error logging for write, resize, kill, pause, resume, and non-Error values
• All 19 PTY tests pass
• pnpm check:all passes (lint + coverage + build)

🤖 Generated with Claude Code

[xiaolai]

✅ VERIFIED — All 7 silent .catch(() => {}) blocks in src/lib/pty.ts are replaced with appropriately-leveled logging (ptyWarn for write/resize, terminalLog for kill/pause/resume/close), with proper error narrowing and matching test coverage.