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

docs/src/app/commands/page.mdx

@@ -177,6 +177,8 @@ agent-browser network unroute [url]            # Remove routes
 agent-browser network requests                 # View tracked requests
 agent-browser network requests --clear         # Clear request log
 agent-browser network requests --filter <pat>  # Filter by URL pattern
+agent-browser network har start                # Start HAR recording
+agent-browser network har stop [output.har]    # Stop and save HAR (temp path if omitted)
 ```
 
 ## Tabs & frames

docs/src/app/security/page.mdx

@@ -169,7 +169,7 @@ Example policy (restrictive):
     <tr><td><code>wait</code></td><td>wait, waitforurl, waitforloadstate, waitforfunction</td></tr>
     <tr><td><code>get</code></td><td>get text/html/url/title, count, isvisible, getbyrole, getbytext, getbylabel, etc.</td></tr>
     <tr><td><code>interact</code></td><td>hover, focus, drag, press, keydown, keyup, mousemove, dispatch</td></tr>
-    <tr><td><code>network</code></td><td>network route/unroute, requests</td></tr>
+    <tr><td><code>network</code></td><td>network route/unroute, requests, har start/stop</td></tr>
     <tr><td><code>state</code></td><td>state save/load, cookies set, storage set</td></tr>
   </tbody>
 </table>