fix(pr): skip linguist-generated files before highlighting

[floze-the-genius]

Fixes #588.

What changed

• Resolve linguist-generated through Git's attribute matcher when a PR has a matching local checkout.
• Exclude generated files in the unified-diff parser before their hunks are materialized or syntax highlighted.
• Use the same filtered path for initial PR loading and commit-range reloads.
• Document the local-checkout behavior and cover true, bare, false, and unspecified attribute values.

This is intentionally narrower than #525: that PR adds optional post-load collapsing and review-progress behavior. The change here addresses the #588 stall specifically by preventing generated files from reaching syntax highlighting at all.

Verification

• Regression test failed before the fix (2 files loaded instead of 1) and passes after it; the retained Rust file still has highlighted spans.
• cargo check
• cargo fmt --all --check
• cargo clippy -- -D warnings
• cargo test (1473 passed, 0 failed, 2 ignored)
• Dogfooded with cargo run -- --no-update-check against this branch's own six-file diff.

AI assistance

Implemented and reviewed with OpenAI Codex. I verified the behavior, tests, full diff, and final commit locally.