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/dsl/grammar/mod.rs

@@ -613,6 +613,7 @@ pub static REGISTRY: &[(&CommandNode, CommandCategory)] = &[
     (&app::MESSAGES, CommandCategory::Simple),
     (&app::UNDO, CommandCategory::Simple),
     (&app::REDO, CommandCategory::Simple),
+    (&app::COPY, CommandCategory::Simple),
     (&ddl::DROP, CommandCategory::Simple),
     (&ddl::ADD, CommandCategory::Simple),
     (&ddl::RENAME, CommandCategory::Simple),