fix: fail fast on invalid baseline skills

[steezkelly]

Summary

Partially addresses #33 H3 by making baseline constraint failures a hard gate:

• adds _require_constraints_pass(...) to fail fast on any required constraint failure
• uses that gate for baseline skill validation before optimizer setup
• validates the complete baseline skill (skill["raw"]) instead of body-only markdown so skill_structure sees frontmatter
• adds regression tests for raw baseline validation and fail-fast constraint behavior

Root cause

The pipeline previously logged a warning when the baseline skill failed constraints, then continued optimization anyway. That made improvement metrics unreliable because the candidate was compared against a malformed baseline.

A direct hard-fail would have exposed a second issue: the old baseline validation used skill["body"], but skill_structure requires YAML frontmatter. This PR therefore validates the full raw skill file before enforcing the gate.

Opposite-perspective review notes

I specifically checked the failure mode that could make this PR harmful:

• Naive hard-fail on the existing body-only validation would reject every valid skill as missing frontmatter.
• The implementation avoids that by validating skill["raw"] for the baseline.
• Evolved-skill full-file validation is still a separate upstream issue covered by the existing Constraint validator rejects every evolved skill: checks frontmatter on body-only text #11/Bug: validate_all checks evolved_body instead of evolved_full, causing all evolved skills to fail skill_structure #34/fix: extract evolved skill body from optimized predictor #49/fix: validate assembled skill (evolved_full) not raw body (evolved_body) #50/fix: load_skill handles dir paths; validate evolved_full not body #51 family of PRs; this PR is scoped to baseline gating.

Test Plan

• RED first: pytest tests/skills/test_evolve_skill_constraint_gates.py -q failed because _require_constraints_pass and _validate_baseline_constraints did not exist
• pytest tests/skills/test_evolve_skill_constraint_gates.py -q
• pytest -q
• static added-line security scan
• git diff --check

Result: 142 passed, 11 warnings (DSPy deprecation warnings only).

Partially addresses #33 (H3: evolution proceeds despite baseline constraint violations).

[steezkelly]

Closing this split PR in favor of consolidated PR #67. Local integration found review/merge overhead across the stack (notably #61/#64 overlap in evolution/skills/evolve_skill.py), and #67 preserves the combined local test evidence: targeted stack tests 21 passed; full suite 160 passed; GitHub checks were absent on the split PRs. Review #67 instead.