Rewrite skills README and harden release guidance

[Yuge Zhang (ultmaster)]

Summary

This draft combines the skills README rewrite with the release-skill corrections from #555.

Skills README rewrite

• lead with what the skill does for a coding agent
• explain how the optimizer harness and Agent Lightning skill work together
• clarify the experiment setup, coding-agent rows, budget control, and performance breakdowns

Release skill hardening

• warn that failed uv version bumps can leave partial version state
• correct the CI test-set and runtime-version explanations
• make merge selection and tag-workflow lookup deterministic and noninteractive

Verification

• skill-creator quick_validate.py
• skills-ref==0.1.1 Agent Skills validation
• git diff --check
• confirmed the README content is unchanged from the original Rewrite skills README and harden release guidance #552 head
• confirmed the release skill is unchanged from the validated Harden release skill guidance #555 head

Existing README review items

• [agent-lightning] is used as a reference-style link but has no matching definition.
• The finale-cost paragraph says every point is an average of three runs; confirm whether the charts instead plot individual replicates.
• Confirm whether the charts still need the ALFWorld $0 finale-cost/jitter note, dotted score-only baseline, and pristine-baseline reference points.
• Confirm whether to retain the note that skills/agent-lightning/ is both the canonical Agent Skills package and Claude Code plugin root.
• The file starts at ## with no # heading.
• Several pre-existing wording and typo fixes remain open.

Supersedes #555.