Adds a `statusline.show_last_command` config toggle (default: false) that
appends ` │ last: /<cmd>` to the statusline, showing the most recently
invoked slash command in the current session.
The suffix is derived by tailing the active Claude Code transcript
(provided as transcript_path in the hook input) and extracting the last
<command-name> tag. Reads only the final 256 KiB to stay cheap per render.
Graceful degradation: missing transcript, no recorded command, unreadable
config, or parse errors all silently omit the suffix without breaking the
statusline.
Closes#2538
533973700c