Fix docs-only gate for binary files

[Svaag]

Summary

• change the docs/text validation gate to read only mutated text paths instead of every file in the worktree
• keep the gate strict for non-UTF-8 mutated docs
• add focused gate-runner tests

Why

A production Engineering Loop canary on loop created the requested docs change, but the generic docs gate scanned every file in the repo and failed on an unrelated binary file. This blocked draft PR publication even though the changed doc was valid.

Validation

• uvx ruff check src tests
• uv run mypy --strict src
• uv run pytest

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve colons in local docs paths

When a changed doc path itself contains a colon (valid in git paths on POSIX, e.g. docs/foo:bar.md from backend changed_paths), the normalization above treats it like a repo:path key and this new targeted list serializes only bar.md. The generated gate then sees bar.md missing and continues, so the actual mutated doc is never UTF-8 checked; a non-UTF-8 docs/foo:bar.md now passes even though the previous rglob scan failed. Please only strip repo prefixes for repo-scoped mutation keys, not arbitrary colon-containing paths.

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