docs(AGENTS.md): add detailed documentation for AGENTS.md files in subdirectories

docs/skills/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# skills
+
+## Purpose
+The skill inventory that feeds the Skills column of every résumé variant. Three files, three different shapes.
+
+## Key Files
+| File | Shape | Description |
+|------|-------|-------------|
+| `languages.yml` | Tiered map | `expert` / `strong` / `familiar` lists of programming languages. |
+| `frameworks.yml` | Flat list, `#`-commented groups | Mobile / Web / Blockchain / Infrastructure sections in one sequence. |
+| `methodology.yml` | Flat list | Practices and disciplines (CI/CD, TDD, System Architecture, …). |
+
+## For AI Agents
+
+### Working In This Directory
+- Keep skill names spelled canonically and consistently with `../experience/*.yml` and `../projects/*.yml` so they match across the corpus.
+- `frameworks.yml` uses YAML comments (`# Mobile`) as visual grouping only — they are not data; preserve them when editing.
+- `languages.yml` tiers are subjective rankings; don't promote/demote without reason.
+
+### Testing Requirements
+```bash
+python3 -c "import yaml; yaml.safe_load(open('<file>.yml'))"
+```
+
+## Dependencies
+### Internal
+- Consumed by `../../src/*.tex` (Skills column) by hand. No generation.
+
+<!-- MANUAL: Any manually added notes below this line are preserved on regeneration -->