Add GUI and TUI session handoff via resume id

[gaoyu06]

A GUI agent session can continue in the native TUI by resume session id, then return to the GUI. Same mosaic chat:<id> tile — not a new create-tab type.

Scope

• Rust validate_tui_args allows claude --resume <id> and codex resume <id> (id still is_valid_session_id). JuCode still has no extra argv; the TUI gets /resume <id> on stdin after spawn.
• Session surface: 'gui' | 'tui'. GUI engine is closed before TUI resume; pty is closed before GUI respawn.
• Composer footer: 「在 TUI 中继续」 for jucode/claude/codex when a valid session id exists. ACP never hands off.
• Standalone persisted tui:* tabs are unchanged. Mosaic + does not regain TUI create options.

Ship-ready

• pnpm test: 30 files / 477 tests.
• pnpm check: clean.
• cargo test: 121 passed; clippy clean.

Needs review

• Exclusive ownership of the conversation (no dual process).
• Restore-into-TUI vs return-to-GUI resume paths.
• JuCode stdin /resume timing.

Review preconditions

• Claude: GUI → claude --resume <uuid> → back to GUI with engine resume.
• Codex: resume <threadId> similarly.
• Confirm ACP has no TUI control.
• Confirm empty new chats cannot hand off.