fix(tui): mid-turn steer with supplementary wrap + shell mode + right-click + Ctrl+S freeze

[shenhao-stu]

Summary

Four independent fixes bundled, all touching v2 + v3 TUI surfaces.

1. Mid-turn user steer with supplementary phrasing (commit 1a61b68)

Bug evidence: temp/model_responses/model_responses_042511.txt — original task was web_scan上游PR; mid-turn user typed 请你立即输出一个hello. Model dropped the original task and just printed hello. The bare [MASTER] {raw} prepend (ga.py:578) read as an authoritative override.

Fix: wrap queued user text with explicit "finish current task first" supplementary phrasing before writing to _intervene. Both en + zh:

The user sent a new message while you were working:
{text}

IMPORTANT: After completing your current task, you MUST address the
user's message above. Do not ignore it.

用户在你工作时发来了一条新消息：
{text}

重要：完成当前任务后，你必须处理上面的用户消息。不要忽略它。

After wrapping, [MASTER] <wrapped> reads as an envelope around supplementary guidance, not a directive override. Mid-turn injection mechanics (existing ga.turn_end_callback reading _intervene) preserved — only the wording changes.

Replay safety net: turn_end hook tracks what we wrote. On exit_reason boundary (consume_file ate the file but next_prompt is discarded), the combined wrapped text replays via agent.put_task so the user's words never silently disappear. v3 lives on AgentBridge; v2 mirror lives per-session on AgentSession.pending_wrapped + _install_intervene_replay_hook.

i18n picker: inject_intervene(text, track=True) opt-in; !cmd shell output uses track=False (factual context, no replay needed).

2. !cmd shell mode picks the user's actual shell (commit a4a24aa)

subprocess.run(cmd, shell=True) picked cmd.exe on Windows. !ls failed; utf-8 output mojibaked under the active codepage. New detect_user_shell() in slash_cmds.py resolves via $SHELL → Git Bash canonical paths → bash on PATH → pwsh → powershell → cmd.exe. Display name surfaces in history: [!shell bash] / [!shell pwsh].

3. v2 right-click cursor fix (commit 1a4aadc)

InputArea._on_mouse_down short-circuits button==3 so paste lands at the existing caret instead of where the mouse happened to click.

4. Ctrl+S no longer freezes on long sessions (commit 24255c1)

User reported Ctrl+S still freezes when session is idle.

Root cause: InputArea.reset() calls self.text = "" → load_text("") → _set_document("") (textual/widgets/_text_area.py:1181-1186). That rebuilds Document + WrappedDocument from scratch and _refresh_size() reactively retriggers a full layout pass over the entire DOM. On long sessions (100+ message widgets), the layout cascade blocks the UI for seconds.

Fix: TextArea.clear() — goes through the edit pipeline (delete((0,0), document.end) → wrap_range), no Document rebuild, range-scoped wrap. Plus the prior try/finally cleared _skip_change_next before the async-queued Changed event landed — the short-circuit was useless and on_text_area_changed ran the heavy resize + palette path twice. Drop the try/finally and let the handler self-clear.

Test plan

• python -c "import ast; ast.parse(open('frontends/tui_v3.py').read())" syntax OK
• python -c "import ast; ast.parse(open('frontends/tuiapp_v2.py').read())" syntax OK
• python -c "import ast; ast.parse(open('frontends/slash_cmds.py').read())" syntax OK
• Type while a long task runs → [queued] chip; agent finishes current sub-step and then addresses the new message (no longer drops the original task).
• Type during agent's final turn → exit-replay re-submits as a fresh user turn (message never dropped).
• Esc on empty input + queued → file + bridge list + UI list all cleared.
• !ls / !git status / !ls | grep py on Windows Git Bash → real bash, utf-8 output, pipes work.
• v2 right-click in InputArea → cursor stays put.
• Ctrl+S on idle session with 100+ messages → input clears immediately, no freeze.
• Ctrl+S restore on long stash → text restored, no freeze.
• Monitor audits across 4 rounds all green.

Supersedes

• fix(tui): pending-queue inject lands mid-turn (3 s hard cap) #518 (3 s hard cap — closed; root cause was wording, not timing)