Document Splunk Enterprise admin advisor skill

[cursor]

Summary

• Added /splunk-enterprise-administration-advisor to the README and docs index skill entry points.
• Updated the developer guide with the skill intent, read-only boundaries, evidence model, and vendored Apache-2.0 license note.
• Updated the graduation inventory of tracked Cursor project skills.

Codepaths covered

• .cursor/skills/splunk-enterprise-administration-advisor/SKILL.md
• .cursor/skills/*/SKILL.md project skill maintenance workflow
• Cursor documentation entry points in README.md, docs/README.md, docs/poc/DEVELOPER_GUIDE.md, and docs/poc/GRADUATION.md

Knowledge gaps addressed

• Public docs no longer imply only /usage and /demo-prep are tracked project skills.
• Contributors now have a concise boundary for the read-only Splunk Enterprise administration advisor and know to preserve its vendored license.

Verification

• pre-commit run --all-files
• make test