fix(web): label only the first prompt row with the working directory

A multi-line command repeated the cwd label on every prompt row, which
states something the view does not know: it carries ONE working directory —
where the call started — and a `cd` in the command moves later lines
elsewhere. `cd ~` then `ls` rendered both rows labelled with the session
workspace while `ls` actually listed the home directory.

The label now appears on the first row only, and later rows keep a bare `$`
so they still read as prompts. Same reasoning as the run-state dot: neither
a per-line directory nor a per-line exit status exists to report.

The built-bundle snapshot records the effect on fixture turn 60's two-line
command (`fixture echo done` becomes `$ echo done`).

apps/web/tests/terminal-card.snapshot.ts

@@ -264,7 +264,7 @@ it('the fallback row reaches the same card through its expand control', async ()
       ],
       "prompt": [
         "fixture ls -la",
-        "fixture echo done",
+        "$ echo done",
       ],
       "runState": "done",
       "runStateLabel": "已完成",