Centralize Changed File projections

[dbrosio3]

This pull request introduces a set of improvements and refactorings to the path policy and AI review modules, focusing on more precise handling of changed files, especially around live (non-deleted) files and changed line metrics. The changes add new helper utilities for selecting live files, counting changed text lines, and projecting file paths, and they update both internal logic and documentation to reflect these enhancements.

Path policy and changed file utilities:

• Added new helpers in src/path-policy/filtering.ts for selecting live changed files (selectLiveChangedFiles), projecting their paths (selectLiveChangedFilePaths), and counting changed text lines (countChangedTextLines). These utilities improve accuracy and code clarity when working with changed files. [1] [2]
• Updated selectToolChangedFilePaths to use the new selectLiveChangedFilePaths helper, ensuring only live files are considered for tool commands.

AI review and guardrails integration:

• Refactored src/ai/guardrails.ts and src/ai/review-context.ts to use countChangedTextLines and selectLiveChangedFiles, ensuring binary and deleted files are handled correctly in AI guardrail checks and when collecting full-file context. [1] [2] [3] [4] [5]

Policy enforcement improvements:

• Updated built-in policy logic in src/runner/policies.ts to use the new helpers for counting changed lines and filtering live files, improving accuracy and consistency in diff size and forbidden path checks. [1] [2] [3]

Documentation updates:

• Revised docs/architecture/modules.md to document the new projections, metrics, and helper functions, reflecting the expanded capabilities of the path policy module. [1] [2] [3]

Testing enhancements:

• Added and updated tests in test/path-policy.test.ts to cover the new helpers and ensure correct behavior for live file selection and changed line counting. [1] [2]
• Minor test fixture cleanup in test/ai.test.ts to align with new logic.