docs: restructure skill catalog and standardize setup-skills references

Reorganize README.md with category sections (engineering, pkm, etc.)
for both user-invoked and model-invoked skills. Add new subcategory
READMEs for skill-authoring and thinking-and-docs. Replace
/setup-matt-pocock-skills references with setup-skills across all SKILLs.

skills/thinking-and-docs/README.md

@@ -0,0 +1,20 @@
+# Thinking and Docs Skills
+
+Clarify decisions, capture ideas, and improve project documentation.
+
+## User-invoked
+
+- [before-building](before-building/SKILL.md) — Surface consequential choices when the user proposes a build.
+- [decisions](decisions/SKILL.md) — List choices made during the current work that remain uncertain.
+- [level-up](level-up/SKILL.md) — Assess technical and product knowledge and grow a learning plan.
+- [read-all-adrs](read-all-adrs/SKILL.md) — Read every ADR in the project's `docs/adr/` folder.
+- [remind](remind/SKILL.md) — Rewrite the last response more simply and briefly.
+- [short](short/SKILL.md) — Compress the current answer while keeping its substance.
+- [teach](teach/SKILL.md) — Teach the user a new skill or concept within the workspace.
+
+## Model-invoked
+
+- [brain-to-docs](brain-to-docs/SKILL.md) — Extract project vision, decisions, and preferences into documentation.
+- [next-decision](next-decision/SKILL.md) — Drill into the next unresolved decision with choices and a recommendation.
+- [prompt-me](prompt-me/SKILL.md) — Ask pointed questions to extract project priorities and concerns.
+- [save-idea](save-idea/SKILL.md) — Capture content ideas in the user's content backlog.