fix(pr-review): paginate complete file evidence

[BigDataDZ]

Summary

• compare the GraphQL detail page against the scalar changedFiles count
• fetch all changed files through the paginated REST endpoint when the detail page is truncated
• fail closed when the paginated result still does not match the authoritative count
• move GitHub source reads into the existing PR-review capability boundary so the hot command module stays within its maintainability budget

Validation

• pytest -q tests/test_pr_review_github_scan.py tests/canary/test_maintainability_ratchet.py — 18 passed
• ruff check on all changed Python files — passed
• python examples/pr-review-command-smoke.py — passed
• real microsoft/vscode#334528 REST pagination — 822/822 files returned
• public-boundary scan — clean
• standard premerge gate — passed, including maintainability ratchet

This is a focused follow-up to huangruiteng#4077 and targets its head branch directly.