feat: shell integration (OSC 133)

Terminals now pick up OSC 133 prompt/command/output/finished marks,
which the VT parser ignores: a scanner (app/src/osc.rs, split-safe
across reads, bodies capped at 4 KiB) finds them in the output stream
and each mark records the cursor line. Lines are stable indexes that
account for lines a full scrollback rotates out, so marks stay on their
text; clearing the scrollback drops them.

With marks present: Ctrl/Cmd+Shift+Up/Down and Previous/Next Prompt in
the context menu jump between prompts, Copy Last Command Output copies
what the last finished command printed, and a green/red bar beside each
prompt shows the exit status (hover for code and duration). OSC 7
working-directory reports are parsed too (used by the next change).
The manual shows bash and zsh snippets; fish 4 and common prompt themes
already send the marks.

rust/app/src/app.rs

@@ -846,7 +846,8 @@ impl App {
                     t.reset();
                 }
             }
-            MenuAction::Copy | MenuAction::SelectAll => {}
+            // Handled inside the terminal view.
+            MenuAction::Copy | MenuAction::SelectAll | MenuAction::PrevPrompt | MenuAction::NextPrompt | MenuAction::CopyOutput => {}
         }
     }