feat: support custom headers for CDP connect

[giulio-leone]

Closes #396

Summary

Adds support for custom HTTP headers when connecting to a browser via CDP (Chrome DevTools Protocol).

Changes

• src/browser.ts: connectViaCDP() now accepts an optional headers parameter and applies them to all browser contexts after connection via context.setExtraHTTPHeaders()
• src/daemon.ts: Parses AGENT_BROWSER_HEADERS env var (JSON string) and passes it to the launch command
• cli/src/connection.rs: ensure_daemon() accepts a headers parameter and forwards it as AGENT_BROWSER_HEADERS env var to the daemon process (both Unix and Windows)
• cli/src/main.rs: Passes flags.headers to ensure_daemon()
• cli/src/flags.rs: Initializes headers from AGENT_BROWSER_HEADERS env var as fallback (CLI --headers flag takes precedence)

Usage

# Via CLI flag
agent-browser --headers '{"Authorization": "Bearer token"}' open example.com

# Via environment variable
AGENT_BROWSER_HEADERS='{"Authorization": "Bearer token"}' agent-browser open example.com

Note on Playwright limitation

Headers are applied post-connection, not during the WebSocket handshake. Playwright's connectOverCDP() does not support connection-time headers, so we set them on the browser context immediately after the CDP connection is established using context.setExtraHTTPHeaders(). This means headers will be included in all subsequent HTTP requests made by pages in the connected browser, but not in the initial CDP WebSocket connection itself.

[vercel]

@g97iulio1609 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[vercel]

Additional Suggestion:

Custom headers from --headers flag are silently dropped when using --auto-connect mode because autoConnectViaCDP() doesn't receive or forward headers to connectViaCDP().