Add --profile flag for persistent browser profiles (#68)

* 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.

* Expand tilde in profile path to home directory

* fix: add missing profile field to test Flags struct

---------

Co-authored-by: Chris Tate <chris@ctate.dev>

src/types.ts

@@ -17,6 +17,7 @@ export interface LaunchCommand extends BaseCommand {
   cdpPort?: number;
   cdpUrl?: string;
   extensions?: string[];
+  profile?: string; // Path to persistent browser profile directory
   proxy?: {
     server: string;
     bypass?: string;