Add --profile flag for persistent browser profiles

Adds support for persistent browser profiles that preserve cookies,
localStorage, and login sessions across browser restarts.

Changes:
- Add --profile <path> CLI flag (flags.rs)
- Add AGENT_BROWSER_PROFILE environment variable support
- Add profile field to LaunchCommand type (types.ts)
- Use launchPersistentContext when profile is specified (browser.ts)
- Update help text and README with documentation

Usage:
  agent-browser --profile ~/.myapp-profile open myapp.com

This enables AI agents to maintain authenticated sessions across
browser restarts without re-authenticating each time.

cli/src/output.rs

@@ -1189,6 +1189,7 @@ Snapshot Options:
 
 Options:
   --session <name>           Isolated session (or AGENT_BROWSER_SESSION env)
+  --profile <path>           Persistent browser profile (or AGENT_BROWSER_PROFILE env)
   --headers <json>           HTTP headers scoped to URL's origin (for auth)
   --executable-path <path>   Custom browser executable (or AGENT_BROWSER_EXECUTABLE_PATH)
   --extension <path>         Load browser extensions (repeatable).
@@ -1207,6 +1208,7 @@ Examples:
   agent-browser get text @e1
   agent-browser screenshot --full
   agent-browser --cdp 9222 snapshot      # Connect via CDP port
+  agent-browser --profile ~/.myapp open example.com  # Persistent profile
 "#
     );
 }