chore: refresh project agent guidance

Clarify template guidance, move skills, and document formatter and
commit conventions.

Co-authored-by: GPT-6 Luna <gpt.6.luna@openrouter.ai>

.agents/README.md

@@ -0,0 +1,39 @@
+# Agent Skills
+
+This directory contains reusable, agent-agnostic guidance for common
+project tasks. The skills are Markdown documents, not instructions tied
+to a particular coding-agent product. Read the relevant skill when a
+task matches its description; skill discovery and loading support varies
+between tools.
+
+## Skills
+
+- [`tdd`](skills/tdd/SKILL.md): Test-driven development with Unity and
+  CMock.
+- [`cmake`](skills/cmake/SKILL.md): Build configuration, modules, and
+  dependencies.
+- [`git`](skills/git/SKILL.md): Commit messages, tagging, and branching.
+
+## Adding a Skill
+
+Place a `SKILL.md` inside a named directory under `.agents/skills/`:
+
+```
+.agents/skills/
+  my-skill/
+    SKILL.md
+```
+
+Include frontmatter with the skill's name and a short description of
+when to use it:
+
+```yaml
+---
+name: my-skill
+description: Describe the tasks this skill helps with.
+---
+```
+
+Keep the instructions self-contained and useful across coding-agent
+tools. Avoid tool-specific loading commands unless a skill is explicitly
+written for that tool.