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.

docs/overview.md

@@ -73,6 +73,7 @@ Use Nexus Terminal when you need to:
 - Shift+PageUp/PageDown/Home/End and the mouse wheel scroll the scrollback; on the alternate screen the wheel sends arrow keys, so `tmux`, `screen` and `byobu` scroll naturally.
 - Context menu with Copy, Paste, Select All, Find, Send Snippet, Port Forwards, Split Right, Split Down, Export Output, Clear and Reset.
 - Export the whole scrollback as plain text.
+- Shell integration (OSC 133): jump between prompts, copy the last command's output, and see each command's exit status and duration beside its prompt.
 - Status-bar latency for the active SSH session.
 
 ### SSH capabilities