- Add extract_file_paths_from_diff() to parse diff headers
- Inject 'Valid files in this diff:' list into review user prompt
- Add post-parse filtering of issues with invalid file paths in review.rs
- Add is_valid_file_path() with exact + partial match heuristic
- Add 7 new tests (file path extraction, prompt injection, dedup, /dev/null)

Anti-hallucination: LLM can now see exactly which files are valid,
and issues referencing non-existent files are filtered out post-parse.

Closes #93

- Refactor extract_file_paths_from_diff: early return pattern, no loop
  over prefixes (fixes overly broad matching of '--- ' prefix)
- Tighten is_valid_file_path: require '.' in suffix match to avoid
  matching single-char paths like 'a' or 'b'
- Addresses cora review findings (major x2)

- Extract basename from issue file path and compare against valid file basenames
- Prevents false matches like '.rs' matching 'main.rs'

Simplify to exact match — fuzzy matching introduces false positives.
The prompt now includes 'Valid files in this diff:' list so the LLM
should use those exact paths.

- Add ReviewSection and ScanSection to CoraFile schema
- Add response_format, review/scan system_prompt fields to Config
- Wire response_format to LLM API calls (chat_completion and streaming)
- Support custom system_prompt and system_prompt_file for review and scan
- Update all callers in review.rs, scan.rs, and llm.rs

Refs #92 #94

- Config defaults for new fields (response_format, prompt overrides)
- ReviewSection YAML parsing and merge (response_format, system_prompt, file)
- ScanSection YAML parsing and merge
- Full config file with both review and scan sections
- resolve_system_prompt priority logic (inline > file > none)
- system_prompt_file resolution from disk
- Missing file handling (returns None, logs warning)

Refs #92 #94

- Canonicalize path and check it's within project root (CWD)
- Block paths outside project root (e.g. /etc/shadow, ../../.env)
- Fix test to use project-local file instead of temp dir
- Add test for path traversal rejection

Addresses cora review finding: MAJOR arbitrary file read via config.

Security: prevents secret leakage through malicious .cora.yaml commits.

- Replace .ok()? with explicit match for canonicalize error
- Log debug message when file doesn't exist (intended behavior)
- Remove accidentally duplicated project_root line

Both the file path and project root are now canonicalized before comparison,
preventing potential bypass via symlinked project root components.

Addresses cora review finding: symlink bypass in path traversal guard.

Added:
- Custom system prompts via .cora.yaml config (#94)
- response_format: json_object opt-in (#92)
- File path injection into review prompts (#93)
- Post-parse file path filtering (#93)
- Enhanced default system prompts (#95)

Fixed:
- Path traversal in system_prompt_file (#92)
- Symlink bypass in path traversal guard