Expose HAR recording as a CLI subcommand under the existing `network`
command so users can capture and export network traffic without a
separate tool or opening the browser twice.
- Parse `network har start` and `network har stop [path]` in commands.rs
- Enrich HarEntry with request/response headers, timestamps, status text,
resource type, HTTP version, and body sizes from CDP events
- Produce HAR 1.2 output with creator/browser metadata, query strings,
and proper header arrays compatible with Chrome DevTools HAR viewer
- Auto-generate output path under ~/.agent-browser/tmp/har/ when omitted
- Add har_stop to skip_launch list so export works without a live browser
- Update help text, README, docs site, SKILL.md, and security policy docs
- Add unit tests for parsing, HAR entry serialization, and stop behavior
