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.

docs/requirements.md

@@ -385,6 +385,15 @@ since ADR-0027.)
   redisplaying schema info on demand. *(Progress: `show table
   <name>` and `show data <Table>` implemented;
   `show tables`, `show relationships`, etc. pending.)*
+- [x] **V6** Copy the output panel to the system clipboard
+  (issue #11, ADR-0041). `copy` / `copy all` copy the whole
+  panel; `copy last` copies the most recent command's output.
+  Delivery is OSC 52 (SSH-friendly, no native dep) *plus* a
+  best-effort native write (`arboard`), always both; the payload
+  is the panel's plain text exactly as rendered. Removes the
+  terminal-select-and-fight-wrapping friction of filing bug
+  reports. (Complements V4's planned Markdown export — a
+  different "get the session out" path.)
 
 ## Project lifecycle (per ADR-0004)