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.

src/completion.rs

@@ -1512,7 +1512,7 @@ mod tests {
         // commands in the entry-keyword set.
         for expected in &[
             "quit", "help", "rebuild", "save", "new", "load", "export",
-            "import", "mode", "messages", "undo", "redo",
+            "import", "mode", "messages", "undo", "redo", "copy",
         ] {
             assert!(
                 cs.contains(&expected.to_string()),