Add authoritative local validation gate

[jwalin-shah]

Summary

• add a single local validation gate at python3 tools/local_validation.py
• document the gate in worker-facing validation docs and project context
• extend packaging/CI contract tests to prove the gate runs executable checks and exits nonzero on failures

Validation

• python3 -m pytest tests/test_packaging_ci.py -q -> 13 passed
• python3 -m pytest -q -> 209 passed
• git diff --check -> passed
• python3 tools/local_validation.py -> 209 passed, git diff --check passed

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[jwalin-shah]

Portfolio monitor review found a blocking follow-up before merge: Codex review flagged P2 that the canonical local handoff gate checks only unstaged worktree changes with git diff --check, so staged/committed PR whitespace errors can pass. Please include staged/committed changes or compare against the PR base/merge-base before merge.