fix: reduce false positives in SARIF review

[ajianaz]

What

Three improvements to reduce false positives in cora's review output:

1. Context line filter — Drops Minor and Info findings on unchanged (context/removed) diff lines. These are pre-existing code that appeared in the diff due to surrounding changes, not new code introduced by the PR.

2. Enhanced system prompt — Added LANGUAGE-SPECIFIC FALSE POSITIVE AWARENESS section to the review system prompt:

   • In-place mutation methods (Vec::retain, Vec::append, Vec::sort, etc.) should not be flagged as "result ignored"
   • Error/early-return paths (Err => return) should not be flagged for missing happy-path post-conditions

3. SARIF non-blocking — Fixed cora-review-action (separate PR) so Check for blocking issues step respects upload-sarif: 'false' input.

Why

Issue #507 identified recurring false positive patterns:

• Pattern Core CLI structure and commands (review, scan, init, auth, hook) #1: In-place mutations flagged as "result ignored"
• Pattern OpenAI-compatible LLM engine (reqwest-based, streaming status) #2: Error path code flagged for missing happy-path post-conditions
• Pattern .cora.yaml config system (per-project, hierarchical) #3: Pre-existing code in diff context flagged as new issues

These false positives erode developer trust in automated review and create noise that masks real issues.

How

• apply_context_line_filter() in review.rs: builds (file, line) -> DiffLineType lookup from parsed diff chunks, then retains only findings on Add lines (or Critical/Major on any line)
• System prompt additions in llm.rs: explicit Rust mutation method list + error path guidance
• cora-review-action/action.yml: gate blocking check behind upload-sarif == 'true'

Testing

• cargo build — clean
• cargo test — 22/22 pass (16 unit + 6 config)
• cargo clippy — 0 warnings
• cargo fmt --check — clean
• Pre-commit cora review — passed

Related Issues

Fixes #507

Checklist

• Code follows project style (cargo fmt)
• No clippy warnings
• All tests pass
• Self-reviewed