Add a fail-closed safe-output gate

[eshulman2]

Summary

• add a centralized fail-closed validation boundary to both Git push paths
• reject protected paths, unsafe symlinks, oversized output, malformed Git metadata, and missing trusted base references
• support global and repository-oriented protected path rules
• add an OutputValidator extension interface for secret scanning and future validation controls
• document output-gate configuration

Security model

Validation compares changes against the trusted origin/HEAD merge base and executes before external push. Invalid policy or validation errors fail closed.

Test plan

• 16 focused output-validation and Git operations tests pass
• Python compilation succeeds
• git diff --check passes
• Run Ruff in CI (the isolated implementation clone could not download a missing dependency because network DNS was unavailable)

Closes #263

Related: #77, #150, #174

[eshulman2]

Closing to refine the implementation as a repository-change validation boundary in GitOperations, with typed results and a clean validator composition point for #77, before reopening.