Copy this turn / last reply, like CLI /copy #493

What the CLIs do

Kimi 0.27: `/copy` copies the last assistant message. Grok has `/copy` in minimal mode. Claude lets you select and copy; the TUI also has copy-last in places.

The intent is: the answer is done, I need it in a PR description / Slack / another thread, I do not want to drag-select a virtualized bubble.

Solenta today

The only copy control in `ThreadView` is "copy thread id." User messages have Edit. Assistant messages are Markdown with no copy button. There is no `/copy` in `ORCH_COMMANDS`. Selecting text works until the next stream tick re-renders the node and kills the selection (#91 class of problem).

Why it matters

Same CLI grammar cluster as #472. People already type `/copy`. A hover copy on the last assistant turn is the GUI spelling.

Scope

1. Hover/focus control on an assistant message: Copy (plain + Markdown of that turn only, not tool cards unless expanded).
2. `/copy` in the Composer / palette should include the CLI verbs, not only orchestration #472 palette copies the latest assistant reply on this thread.
3. Brief toast (reuse the thread-id flash). Do not copy the whole transcript — that is Export a thread as a self-contained, redacted bug report #394.
4. Tests: copy last reply; copy a specific older turn via the button; empty thread is a no-op.

Round 25. Related: #472, #394, #254 (edit user messages).