feat(watch): quick actions — kill pane, abort turn, copy prompt, help overlay

[jiunbae]

Summary

Promote muxa watch from a read-only picker into a command center.
Power users can now act on a selected agent row without leaving the
TUI:

• c — copy the agent's last_prompt to the system clipboard
  (pbcopy → wl-copy → xclip in order, falling back to
  /tmp/muxa-clip-<ts>.txt when none are present).
• K — kill the pane via tmux kill-pane -t <pane_id>.
• R — abort the current turn by sending Ctrl-C to the pane.
• ? — toggle a help overlay listing every keybinding.

Keybindings

Key	Action	Confirm?
c	Copy last prompt to clipboard	no
K	Kill the selected pane	yes
R	Abort current turn (Ctrl-C)	yes
?	Toggle help overlay	no

K / R are capital — Shift required so accidental k (cursor up)
or r (refresh) presses don't blow up a pane or interrupt an agent.

Safety

• Destructive actions gate on a y/N confirm popup that defaults to
  No. Only y / Y / Enter accept; n, Esc, Tab, arrow keys,
  q, even the same key that opened it — anything else cancels.
• Disabled actions (e.g. K on a paneless agent, c on a row with no
  last_prompt) surface a one-line "not applicable" hint in the
  footer rather than silently doing nothing.
• Action results land in a transient ~2 s footer hint
  (✔ killed pane main:2.0, ✗ kill-pane failed: …,
  ✔ copied prompt via pbcopy).

Design choices

• Action dispatch is decoupled from the input handler. A new
  QuickAction enum is what handle_event returns; a separate
  dispatch_quick_action helper executes it via a &mut dyn Effects
  trait. Unit tests inject a RecorderEffects stub instead of
  shelling out to tmux / xclip — fast, hermetic, and lets us cover
  the success / failure / fallback paths.
• No new dependencies. The popup uses the same ratatui
  centered_rect + Clear + Paragraph pattern as the existing
  preview overlay.
• R is "abort current turn", not "restart". The spec called
  for re-launching the original command after Ctrl-C, but we don't
  reliably know that command across the Claude / Codex / Gemini
  wrappers. Pragmatic version is just to send Ctrl-C; users can
  retype the launch.

Test plan

• cargo build --workspace
• cargo test --workspace (15 new tests, all passing)
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• Manual: open muxa watch, navigate to a Claude row, press
  ? and verify the overlay lists every binding
• Manual: press K on a pane-bearing row → confirm popup appears
  → press n → popup closes, pane survives
• Manual: press K again → press y → pane dies, footer shows
  ✔ killed pane … for ~2 s
• Manual: press c on a row with a prompt → footer shows
  ✔ copied prompt via …; verify clipboard contains the prompt
• Manual: press K on a paneless row → footer shows
  kill: no tmux pane on this row

🤖 Generated with Claude Code