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.

Cargo.toml

@@ -10,6 +10,12 @@ publish = false
 
 [dependencies]
 anyhow = "1.0.102"
+# Native system-clipboard for the `copy` command (ADR-0041). Text
+# only: `default-features = false` drops the heavy `image` crate. On
+# Linux this is X11-only — `wayland-data-control` is deliberately
+# omitted (it ~doubles the dep tree), as OSC 52 covers native-Wayland
+# sessions and XWayland covers the rest.
+arboard = { version = "3.6.1", default-features = false }
 base64 = "0.22.1"
 # `clock` brings local-timezone support (UTC → machine local) for
 # the undo-dialog snapshot timestamp (issue #13). No locale feature: