feat(agents): make skill metadata agent-agnostic

AGENTS.md

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+# skills
+
+Personal agent skills packaged through Nix.
+The domain glossary lives in `.agents/CONTEXT.md`.
+
+## Conventions
+
+- `AGENTS.md` and `.agents/` are canonical for agent-facing project instructions, context, ADRs, specs, tasks, and project-local skill links.
+  `CLAUDE.md` and `.claude/` exist only as Claude Code compatibility surfaces.
+- Skill source lives under `skills/<name>/`.
+  Do not edit generated links under `.agents/skills/` or `.claude/skills/` as source.
+- Run `nix flake check` before considering a repository-wide change complete.
+  The check builds every packaged skill plus the Home Manager module, shell hook, and benchmark core tests.
+- The dev shell may place Nix-delivered skills into `.agents/skills/` and maintain `.claude/skills` as a compatibility symlink.
+  Generated ignore entries belong in the root `.gitignore`, not inside `.agents/skills/`, because skill scanners honor ignore files inside scanned skill directories.
+
+## Gotchas
+
+- The vault write area remains `01 sources/claude/` for now.
+  That path is a historical vault taxonomy and is not the canonical project metadata namespace.