Enforce visible-only untrusted extraction

[Driedsandwich]

Summary

Closes #2 when merged.

Related Issue and acceptance criteria

• Issue: [Bug] Enforce visible-only untrusted Issue and Pull Request extraction #2
• Acceptance: rendered visible-only extraction, explicit body/comment separation, trustworthy title/URL metadata, and an untrusted Markdown boundary without permission or network expansion

Changes

• Add rendered-visibility checks with a computed-style and geometry fallback.
• Prefer explicit Issue and Pull Request body containers.
• Keep missing-body fallback distinct from extraction failure; never promote comments or duplicate a body ancestor as a comment.
• Prefer the document title over unrelated global headings.
• Render visible body and comments inside explicit untrusted blockquotes.

Verification

• Focused extractor/formatter suites: 18/18 pass.
• Full concern-only suite: 36/36 pass.
• Hidden, transparent, inert, missing-body, nested-body, unrelated-heading, and Markdown-heading regressions are covered.
• git diff --check passes.
• Codex Security exact diff scan b8153d02-65de-40c2-ba3e-1becf6ccf735: finding 0, complete 2/2 source-like file coverage for 42e6b195...d53ef01.
• Official Gitleaks 8.30.1: finding 0 for the exact d53ef01 tree and introduced range from 42e6b195.
• Exact stacked revision b885b0d verified in isolated headed Chrome for public-safe Issue, Pull Request Conversation, unsupported page, Copy, Download, and manifest permissions.

Risk review

• Permission impact: none; manifest is unchanged.
• Privacy impact: reduces accidental hidden-content capture and marks page content untrusted.
• Dependency impact: none.
• Network or external-communication impact: none.
• Known limitation: rendered visibility remains a best-effort approximation and cannot prove every viewport, clipping, or occlusion state.

Scope check

• No unrelated refactor or formatting change.
• No credentials, private content, personal data, or private screenshots.
• Documentation is isolated in the dependent companion Draft.
• No complete-capture, production-readiness, secret-scanner, distribution, or adoption claim.

Rollback is a revert of this single commit; no persisted data or permission migration is involved. Implemented with Codex under maintainer control. This Draft is not approval to merge or publish.