Implement Linear Issue Bridge

Implement the Linear Issue Bridge as designed in RFD 62.

Summary: A small Go service that serves read-only public pages for Linear issues. Visibility is gated automatically — an issue becomes public when it's referenced in the GitHub repo. The bridge watches GitHub webhook events, detects issue references (e.g. MIR-\d+), and applies a public label in Linear.

Key components:

• GitHub webhook handler to detect issue references in pushes, PRs, comments
• Reference scanner to extract MIR-\d+ patterns from payloads
• Linear API integration to apply public label
• Page renderer serving at linear.miren.garden/{identifier}
• Cache layer with ~5 min TTL on Linear API responses
• Bootstrap script to scan existing repo history and seed labels
• GitHub autolink rule update to point to bridge URLs

Initial scope: mirendev/runtime repo only. Deployed on the garden cluster.