When a local browser_navigate (or any browser command) fails fast because
Chromium isn't on disk, attempt a one-shot binary download via
`agent-browser install` and retry instead of only printing a hint.
Scope is narrow on purpose:
- binary only, never `--with-deps` (that shells apt/needs root, so missing
system libraries stay a user action)
- gated by `security.allow_lazy_installs` (same opt-out as every lazy install)
- skipped in Docker (Chromium ships in the image)
- attempted once per process
Follow-up to #54353, which made the cold-start failure legible; this closes
the "doesn't actually install the missing browser" gap for the common case.
