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.

README.md

@@ -270,6 +270,30 @@ Each session has its own:
 - Navigation history
 - Authentication state
 
+## Persistent Profiles
+
+By default, browser state (cookies, localStorage, login sessions) is ephemeral and lost when the browser closes. Use `--profile` to persist state across browser restarts:
+
+```bash
+# Use a persistent profile directory
+agent-browser --profile ~/.myapp-profile open myapp.com
+
+# Login once, then reuse the authenticated session
+agent-browser --profile ~/.myapp-profile open myapp.com/dashboard
+
+# Or via environment variable
+AGENT_BROWSER_PROFILE=~/.myapp-profile agent-browser open myapp.com
+```
+
+The profile directory stores:
+- Cookies and localStorage
+- IndexedDB data
+- Service workers
+- Browser cache
+- Login sessions
+
+**Tip**: Use different profile paths for different projects to keep their browser state isolated.
+
 ## Snapshot Options
 
 The `snapshot` command supports filtering to reduce output size:
@@ -295,6 +319,7 @@ agent-browser snapshot -i -c -d 5         # Combine options
 | Option | Description |
 |--------|-------------|
 | `--session <name>` | Use isolated session (or `AGENT_BROWSER_SESSION` env) |
+| `--profile <path>` | Persistent browser profile directory (or `AGENT_BROWSER_PROFILE` env) |
 | `--headers <json>` | Set HTTP headers scoped to the URL's origin |
 | `--executable-path <path>` | Custom browser executable (or `AGENT_BROWSER_EXECUTABLE_PATH` env) |
 | `--json` | JSON output (for agents) |