feat: add network har start/stop command for HAR 1.2 export (#874)

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

README.md

@@ -260,6 +260,8 @@ agent-browser network route <url> --body <json>  # Mock response
 agent-browser network unroute [url]            # Remove routes
 agent-browser network requests                 # View tracked requests
 agent-browser network requests --filter api    # Filter requests
+agent-browser network har start                # Start HAR recording
+agent-browser network har stop [output.har]    # Stop and save HAR (temp path if omitted)
 ```
 
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