Remove Brain project planning and brainstorming surface

[JimmyMcBride]

Summary

Remove Brain's built-in project-management surface so the product stays focused on repo-local memory, compiled context, retrieval, session enforcement, and session-scoped distillation.

What Changed

• deleted the brain brainstorm and brain plan command surface
• removed the internal planning, brainstorming, and project packages
• removed the default templates and workspace scaffolding for that surface
• deleted the checked-in .brain/brainstorms/, .brain/planning/, and .brain/project.yaml content from this repo
• simplified brain distill so it is session-only
• removed retired note-type weighting from search
• stopped project workspace detection from treating .brain/project.yaml as active workspace state
• rewrote the README, usage docs, architecture docs, and Brain skill guidance to remove references to the retired workflow
• added durable notes recording the product decision and the repo change

Migration Behavior

• no new project migration is introduced in this PR
• existing Brain repos do not need a migration to keep working
• older repos may still have historical .brain/brainstorms/, .brain/planning/, or .brain/project.yaml files on disk; Brain now ignores that retired surface, and users can keep or delete those files at their discretion

Skill Impact

• skills/brain/SKILL.md is updated to reflect the narrower Brain surface
• the shipped agent descriptors for OpenAI and OpenClaw were updated as well
• installed skill copies outside the repo will pick this up through the normal brain update / reinstall flow

Verification

• go test ./...
• go build ./...