[P1] Sink protocol + GitHubIssuesSink

[thorwhalen]

Pluggable sink (strategy pattern); the v1 default routes a Report to a GitHub issue.

• Sink Protocol: submit(report) -> SinkResult; optional find_duplicates / acknowledge
• Adapter registry — dispatcher never branches on sink type
• GitHubIssuesSink via a GitHub App (short-lived installation token); target repo allowlisted server-side
• Image handling: store screenshot + embed its URL (no GitHub attach API; base64 data-URIs do not render)
• category → labels; env + opt-in logs as collapsible markdown sections

Extra: heed[github]. Design: design.md (Sink interface, GitHub sink) · #1

[thorwhalen]

Phase 1 (#10) landed the core: Sink protocol + BaseSink, StoreSink (default), and GitHubIssuesSink with a dependency-injected create_issue + image-attachment-aware markdown body. Remaining: real GitHub App installation-token brokering (github_sink_from_token is PAT-only so far), server-side repo allow-list resolution, a screenshot-URL resolver, and submit-time dedup.