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

cli/Cargo.toml

@@ -28,6 +28,7 @@ aes-gcm = "0.10"
 async-trait = "0.1"
 similar = "2"
 zip = { version = "8.2.0", default-features = false, features = ["deflate"] }
+time = { version = "0.3", features = ["formatting"] }
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"