feat(tui): add a /copy command to copy the last reply (or transcript) to the clipboard

[liuxiaocs7]

Problem

The terminal TUI has no way to copy conversation content to the clipboard. To
grab an assistant reply a user has to mouse-select it, which is awkward in a
full-screen TUI and breaks entirely over SSH or inside tmux. Every other
surface (Desktop) can copy replies; the TUI cannot.

Proposal

Add a /copy slash command:

• /copy — copy the last assistant reply (plain text) to the system
  clipboard. This is the common case.
• /copy all — copy the whole conversation (user + assistant turns,
  with role labels) as plain text.

Mechanism: OSC 52

Write to the clipboard with the OSC 52 escape sequence
(ESC ] 52 ; c ; <base64> BEL) via the terminal, rather than shelling out to
pbcopy/xclip/wl-copy. OSC 52:

• works across SSH — the sequence rides the same stream as the rendered UI
  and reaches the user's real terminal, not the remote host;
• needs no external binary and no new dependency;
• is the standard approach for terminal-first tools.

tmux is auto-detected (via $TMUX) and the sequence is wrapped in tmux's DCS
passthrough. Note: OSC 52 is fire-and-forget (no acknowledgement), so terminals
that don't support it (e.g. macOS Terminal.app, or tmux without
set-clipboard on) will silently no-op — an accepted limitation of the
protocol.

Behavior details

• Read-only and independent of the running turn (midTurn: 'local'), so it can
  copy the last reply while a new turn streams — same disposition as
  /transcript.
• When there is nothing to copy yet, show a notice instead of writing an empty
  clipboard.
• Localized (en/zh) confirmation notices, following the existing TUI copy
  catalog.

Scope

• TUI only (surfaces: ['tui'], session: 'required').
• New command registered in the slash-command catalog + a handler in the TUI
  runner; a small pure OSC 52 helper and transcript-text helper, both unit
  tested.

[liuxiaocs7]

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