Build browser extension package for relay runtime

[haasonsaas]

Summary

The browser connector and relay session APIs exist in the broker, but there is no browser extension to consume them. The fill flow is fully specified on the server side (selector-bound, single-use, never auto-submit) but has no client.

Required work

• Chrome extension (Manifest V3) that:
  • Registers a relay session with the broker on activation
  • Receives fill instructions (selector/value pairs) from artifact unwrap
  • Injects values into the specified CSS selectors on the target page
  • Never auto-submits forms
  • Shows a confirmation UI before filling
  • Handles single-use enforcement (disable after first fill)
• Extension ↔ broker protocol:
  • Document the message format between extension and broker
  • Implement key exchange for relay session encryption (if the PublicKey field on BrowserRelaySession is intended for this)
  • Handle origin and tab binding verification on the extension side
• Firefox extension (WebExtensions API — share most code with Chrome)
• Security review: ensure the extension doesn't expose credentials to page JavaScript, doesn't persist credentials in extension storage, and validates the broker's TLS certificate

Files

• New directory: extension/ or browser-extension/
• internal/connectors/browser/ — may need protocol refinements based on extension implementation

Priority

Low — the broker-side implementation is complete, but the extension is needed for the browser flow to be usable end-to-end.

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