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/command.rs

@@ -516,6 +516,19 @@ pub enum AppCommand {
     Undo,
     /// Re-apply the most recently undone change, after confirmation.
     Redo,
+    /// Copy the output panel to the system clipboard (ADR-0041).
+    /// `copy` / `copy all` copy the whole panel; `copy last` copies
+    /// the most recent command's output.
+    Copy { scope: CopyScope },
+}
+
+/// Which slice of the output panel `copy` targets (ADR-0041).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CopyScope {
+    /// The entire output buffer (`copy` bare, or `copy all`).
+    All,
+    /// From the most recent echo line to the end (`copy last`).
+    Last,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -871,6 +884,7 @@ impl Command {
                 AppCommand::Messages { .. } => "messages",
                 AppCommand::Undo => "undo",
                 AppCommand::Redo => "redo",
+                AppCommand::Copy { .. } => "copy",
             },
         }
     }