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

skills/agent-browser/SKILL.md

@@ -147,6 +147,12 @@ agent-browser download @e1 ./file.pdf          # Click element to trigger downlo
 agent-browser wait --download ./output.zip     # Wait for any download to complete
 agent-browser --download-path ./downloads open <url>  # Set default download directory
 
+# Network
+agent-browser network requests                 # Inspect tracked requests
+agent-browser network route "**/api/*" --abort  # Block matching requests
+agent-browser network har start                # Start HAR recording
+agent-browser network har stop ./capture.har   # Stop and save HAR file
+
 # Viewport & Device Emulation
 agent-browser set viewport 1920 1080          # Set viewport size (default: 1280x720)
 agent-browser set viewport 1920 1080 2        # 2x retina (same CSS size, higher res screenshots)