docs: Document browser extension feature and API

[brainless]

Summary

Comprehensive documentation is needed for the new browser extension feature, covering both the technical implementation and user-facing instructions.

Technical Documentation

1. Create a new spec file: specs/BROWSER_EXTENSION.md. This document should detail the architecture of the extension, the data flow between the content script, background script, and the manager API, and any key technical decisions.
2. Update Manager Spec: The specs/MANAGER.md file must be updated to include the new /api/v1/projects/{project_id}/feedback endpoint, detailing its purpose, request format, and expected response.

User Documentation

1. Update README: The root README.md should be updated to include a section on the new browser extension. This section should explain:
   • What the extension does.
   • How to install it (build steps or link to a web store).
   • A brief guide on how to use it to capture and send feedback.

Affected Files

• README.md (user-facing documentation)
• specs/MANAGER.md (API documentation update)
• specs/BROWSER_EXTENSION.md (new technical spec)