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/friendly/keys.rs

@@ -186,6 +186,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("help.app.messages", &[]),
     ("help.app.undo", &[]),
     ("help.app.redo", &[]),
+    ("help.app.copy", &[]),
     ("help.ddl.create", &[]),
     ("help.ddl.sql_create_table", &[]),
     ("help.ddl.sql_drop_table", &[]),
@@ -291,6 +292,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.export", &[]),
     ("parse.usage.help", &[]),
     ("parse.usage.import", &[]),
+    ("parse.usage.copy", &[]),
     ("parse.usage.load", &[]),
     ("parse.usage.messages", &[]),
     ("parse.usage.mode", &[]),
@@ -468,6 +470,10 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("mode.show_simple", &[]),
     ("mode.unknown", &["value"]),
     ("mode.usage", &[]),
+    // ---- copy (ADR-0041) ----
+    ("copy.done", &["count"]),
+    ("copy.nothing", &[]),
+    ("copy.unknown", &["value"]),
     // ---- DbError Display fallback ----
     ("db.error.invalid_value", &["detail"]),
     ("db.error.io", &["detail"]),