feat(runneth-default): baseline creative-strategy skills package

[rezakhadjavi]

Summary

Clean staging PR for the runneth-default baseline package (supersedes the older #141/#167 chain, which sat on pre-staging bases).

• Installs analyzing, briefing, creative-generation into /agent/brain/skills/<name>/ (agent_brain root, flat = direct children, which the Harneth skill loader auto-registers).
• Registers the package in package-index.json as categories: ["baseline"], installPolicy: auto, updatePolicy: auto.
• source.ref is main per the package-index validator contract (fix(packages): align package index validation contracts #184); staging serving is handled by the staging builder, not the ref field.
• Adds the core-creative-skills package instruction pointing at the /agent/brain/skills/<name> paths as a discovery backup.

Validation

• Conforms to the current scripts/validate-runneth-package-index.mjs contract on staging (exact manifest/index/source key sets, source.ref === main, agent_brain target root).

Notes

• Nested /agent/brain/skills/<package-name>/<skill>/ layout is intentionally not used: the loader only scans one level (<root>/<skill>/SKILL.md). Tracked for a harness recursion fix in PDEC-9241.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.