docs(skills): one common copy of the skill packs; project keeps blender-director

The 94 symlinks in ~/.agents/skills and the full second copy in
~/.pi/agent/skills/blender-skills were the same packs under two roots, so Pi
reported a collision for every one of them on every start and silently dropped
the loser.

- The 93 upstream packs, the shared references/ notes and the LICENSE now live
  once, flat and one level down, in the common agent folder ~/.pi/agent/skills/.
- The repository keeps only the project-specific pack:
  .agents/skills/blender-skills/blender-director/. Its ../references/ links now
  point at ~/.pi/agent/skills/references/ because it no longer sits beside them.
  Its SKILL.md declared metadata.role twice (specialist, orchestrator), which is
  fatal YAML - the pack had never actually loaded.
- AGENTS.md and Docs/toolchain.md describe the split and warn against copying the
  common packs back into the repository.

Verified by walking all three discovery roots and parsing every frontmatter:
97 skills loaded, 0 malformed, 0 collisions.

AGENTS.md

@@ -179,7 +179,11 @@ Always use a consistent scale test, named export presets, and repeatable import
 
 ## Blender skills
 
-`.agents/skills/blender-skills/` holds 94 Blender skill packs (MIT, blender-skills 1.3.0). They are agent instructions, not add-ons, and nothing in the pipeline depends on them.
+The 93 upstream Blender skill packs (MIT, blender-skills 1.3.0) live once, in the **common** agent folder `~/.pi/agent/skills/`, together with the shared `references/` notes and the upstream `LICENSE`. They are agent instructions, not add-ons, and nothing in the pipeline depends on them.
+
+Only the project-specific pack stays in the repository: `.agents/skills/blender-skills/blender-director/` — the pipeline orchestrator, which names this project's tools and stages. It refers to the shared notes by absolute path (`~/.pi/agent/skills/references/…`).
+
+Do not copy the common packs into the repository: a second copy under a different root is a name collision, and Pi keeps the first one it finds and warns on every start. Add a pack to `~/.pi/agent/skills/<pack>/SKILL.md` (one level down, flat) and it is picked up everywhere; `references/` sits beside them at `~/.pi/agent/skills/references/`.
 
 **All agents must consult the relevant skill packs before starting a task in that discipline.** Each patch section in `TODO.md` lists its suggested skills; the general mapping is: