feat: copy the output panel to the system clipboard (#11)

New app-level `copy` / `copy all` / `copy last` command (ADR-0041).
Delivery is OSC 52 *and* a best-effort native write (arboard), always
both — OSC 52 acceptance is undetectable, so a true fallback can't be
built. Payload is the panel's plain text exactly as rendered (tags,
✓/✗, box-drawing), drift-locked to render_output_line. arboard added
--no-default-features (X11-only; OSC 52 covers Wayland).

Amends ADR-0003's command registry; requirements V6.

docs/adr/0003-input-modes-and-command-dispatch.md

@@ -72,6 +72,7 @@ explicitly; it is not heuristic. The initial canonical list is:
 | `mode`    | Switch between simple and advanced (`mode simple`/`mode advanced`). |
 | `help`    | Show contextual help.                                   |
 | `hint`    | Request a hint for the current input (ADR pending).     |
+| `copy`    | Copy the output panel to the system clipboard (`copy` / `copy all` / `copy last`; ADR-0041, issue #11). |
 | `quit`    | Exit the application.                                   |
 
 This list is **definitive** and applies in both modes. Adding,