fix(setup-changesets): skill quality improvements

[unional]

Summary

• Collapse "Not this skill" block into a single inline sentence; remove external setup-github-repo reference that belongs to a different repo
• Remove ## After setup section (Q13 audit anti-pattern — describes CI pipeline behaviour the agent cannot act on); fold the two actionable constraints into a closing note on Step 7
• Fix lerna migration: keep lerna installed, disable only command.version/command.publish in lerna.json — lerna is a monorepo task runner (lerna run, lerna exec) as well as a release tool, same pattern as nx-release

Test plan

• Verify SKILL.md diff shows only the "Not this skill" block collapse and "After setup" removal
• Verify lerna.md diff shows the keep-installed pattern matching nx-release.md

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5e6f2b4

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