fix(skills): pass evolved_full to skill_structure validator

[jlvitorasso]

Summary

evolve_skill.evolve() calls the constraints validator with evolved_body (the body only), but the skill_structure constraint checks for YAML frontmatter (---), name, and description. That constraint always failed because those live in the frontmatter, not the body.

Result: every evolution — including high-scoring ones — was routed to output/<skill>/evolved_FAILED.md even when the reassembled evolved_full skill was completely well-formed (and got saved as evolved_FAILED.md immediately after, with all the metadata intact).

Fix

Pass evolved_full (assembled one line earlier via reassemble_skill(skill[\"frontmatter\"], evolved_body)) to the validator instead of evolved_body. One-line change.

Repro before fix

python -m evolution.skills.evolve_skill \
  --skill github-code-review \
  --iterations 5 \
  --eval-source synthetic \
  --optimizer-model openai/MiniMax-M2.7 \
  --eval-model openai/MiniMax-M2.7

Output (10 trials complete, optimizer succeeds):

Returning best identified program with score 60.87!
  Optimization completed in 364.3s

Validating evolved skill
  ✓ size_limit: Size OK: 13161/15000 chars
  ✓ non_empty: Artifact is non-empty
  ✗ skill_structure: Skill missing: YAML frontmatter (---), name field, description field
✗ Evolved skill FAILED constraints — not deploying
  Saved failed variant to output/github-code-review/evolved_FAILED.md

evolved_FAILED.md on disk has the full frontmatter + body — confirming the only thing wrong was the validator input.

Repro after fix (same command)

Validating evolved skill
  ✓ size_limit: Size OK: 13481/15000 chars
  ✓ growth_limit: Growth OK: +2.4% (max +20.0%)
  ✓ non_empty: Artifact is non-empty
  ✓ skill_structure: Skill has valid frontmatter (name + description)
  Output saved to output/github-code-review/20260509_224844/
✓ Evolution improved skill by +0.185 (+42.5%)

Test plan

• Run evolve_skill with synthetic eval source on a real skill (github-code-review) — verified output now lands in output/<skill>/<timestamp>/evolved_skill.md with all 4 constraints passing.
• Failed-constraint path (the evolved_FAILED.md save) still uses evolved_full (unchanged at L206), so behavior on truly malformed evolutions is preserved.