feat(runneth-default): install core skills under /agent/brain/skills

[rezakhadjavi]

Summary

Stacks on top of #141. Changes where the runneth-default package stages the three core creative skills.

Before: agent_skills root -> /agent/.agents/skills/<name>/SKILL.md
After: agent_brain root, skills/<name> path -> /agent/brain/skills/<name>/SKILL.md

• analyzing, briefing, creative-generation now install into a dedicated skills/ folder under brain.
• The core-creative-skills package_instruction resource is unchanged (instructions still live in the fixed package instruction path).
• Bumped package version 0.1.0 -> 0.2.0.

Heads-up (the "any issues?" part)

Skills auto-register/auto-invoke only when they live in /agent/.agents/skills/ (or a loaded context layer points at their path). Moving them to /agent/brain/skills/ means they become plain files on disk, not auto-loaded skills. The package_instruction currently only lists the skill names, not paths, so as-is the agent will know the names but not where to read them. If auto-discovery from brain is desired, the instruction file (or a loaded layer) should reference the /agent/brain/skills/<name> path.

Validation

• node --test scripts/validate-runneth-package-index.mjs — agent_brain is an approved target root; relative path skills/<name> passes.