Make local validation gate authoritative

[jwalin-shah]

Summary

• add tools/local_validation.py as the single pre-handoff gate for pytest plus git diff --check
• point CI and worker validation docs at the same gate
• extend packaging/CI contract tests to prove the gate runs executable checks and exits nonzero on pytest failure

Validation

• python3 -m pytest tests/test_packaging_ci.py -q
• python3 -m pytest -q
• git diff --check
• python3 tools/local_validation.py

Residual risk: CI still needs to run on GitHub's Python 3.11 environment; local validation passed on this worktree.

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[jwalin-shah]

Portfolio monitor review found a blocking follow-up before merge: Codex review flagged P2 that tools/local_validation.py uses git diff --check in a way that misses committed PR whitespace errors in CI/clean checkouts. Please compare against the PR base/merge-base or include staged/committed changes before merge.

[jwalin-shah]

Automated repair pass updated this PR, but did not merge it.

Result:

• Repair worker completed and pushed a branch update.
• Fresh Codex review still found blocking issue(s):
• [P2] Check staged edits before passing local validation — /Users/jwalinshah/projects/.workpack-runs/WP-029/tools/local_validation.py:69-69
• [P2] Avoid no-op committed diff checks on main pushes — /Users/jwalinshah/projects/.workpack-runs/WP-029/tools/local_validation.py:37-43

Repair packet: RP-025 for original WP-029
Local review packet: /Users/jwalinshah/projects/reports/review-repair-runs/RP-025.review.md