chore: add pi skills for tdd, cmake, and git

Create .pi/skills/ with three skills:
- tdd: Red-Green-Refactor workflow, Unity assertions, CMock patterns
- cmake: build commands, adding modules, dependency FetchContent pattern
- git: conventional commits, semver tagging, branching conventions

Update AGENTS.md and README.md to reference the skills and template
project description.

.pi/README.md

@@ -0,0 +1,43 @@
+# Pi Skills
+
+Project-level skills loaded by Pi on-demand.
+
+## Skills
+
+| Skill                          | Description                                          |
+| ------------------------------ | ---------------------------------------------------- |
+| [tdd](skills/tdd/SKILL.md)     | TDD workflow with Unity and CMock                    |
+| [cmake](skills/cmake/SKILL.md) | Build configuration, adding modules and dependencies |
+| [git](skills/git/SKILL.md) | Commit messages, tagging, and branching workflow |
+
+## Installation
+
+Skills are auto-discovered by Pi from `.pi/skills/`. No extra setup
+required.
+
+To install this project's skills into another project:
+
+```sh
+pi install -l git:github.com/portersky/helloctdd
+```
+
+Or as a temporary package for the current session:
+
+```sh
+pi -e git:github.com/portersky/helloctdd
+```
+
+Pi auto-discovers `skills/` directories from installed packages.
+
+## Adding a New Skill
+
+Place a `SKILL.md` inside a directory under `.pi/skills/`:
+
+```
+.pi/skills/
+  my-skill/
+    SKILL.md
+```
+
+Pi validates the frontmatter and registers `/skill:my-skill` in the
+TUI.