fix(miniapp): refine PR review inbox behavior

[limityan]

Summary

Refines the built-in PR Review MiniApp so the review workflow stays focused on actionable feedback and the PR detail layout is less jumpy and less tall.

Changes

• Preserve the PR detail scroll position when switching changed files.
• Mark each PR in the left queue as Draft or Ready with distinct chip styling.
• Collapse the overview by default and show only a one-line description preview until expanded.
• Cap the changed-file list height so it no longer stretches the PR detail page too far.
• Change AI draft generation to prefer actionable JSON findings and inline review items; general summary comments are no longer created as selected publishable output by default unless the model explicitly returns a principle-level summaryComment.
• Add focused built-in MiniApp asset tests covering the scroll preservation and compact/actionable review behavior.

Verification

• node --check src/crates/core/src/miniapp/builtin/assets/pr-review/ui.js
• cargo test -p bitfun-core bundled_pr_review -- --nocapture