Default shell terminals to PTY daemon

[ovdmar]

Summary

• default new plain shell terminal sessions to the PTY daemon backend instead of tmux
• keep legacy tmux creation behind CITADEL_TERMINAL_BACKEND=tmux and preserve existing persisted tmux rows
• add best-effort PTY owner cleanup when stopping PTY-daemon sessions

Verification

• pnpm typecheck
• pnpm lint
• pnpm check:arch
• pnpm check:size
• pnpm check:deps
• pnpm build
• pnpm vitest run packages/terminal/src/pty-daemon-client.test.ts packages/terminal/src/pty-daemon-websocket.test.ts
• Earlier targeted operations test passed before the local full-suite Vitest workers entered an unrelated disk-I/O wait. A later rerun under timeout produced no assertion output before timing out, consistent with that local stall.

Scope note

This moves the supported plain shell Terminal path. Agent sessions still use the tmux-specific launch/status/message helpers and need a separate backend-adapter migration before they can become PTY-daemon-backed.