agents: Add contribution guide skill

Define a new contribution guide skill for autonomous agents.
This ensures future automated agent workflows strictly follow the
commit formatting rules defined in docs/commit_messages.md when
submitting changes.

Change-Id: Ia86297d0a465256e352efd82fec6177f18148df6
Reviewed-on: https://bluetooth-review.googlesource.com/c/bluetooth/+/2940

.agents/skills/contribution_guide/SKILL.md

diff --git a/.agents/skills/contribution_guide/SKILL.md b/.agents/skills/contribution_guide/SKILL.md
new file mode 100644
index 0000000..610e712
--- /dev/null
+++ b/.agents/skills/contribution_guide/SKILL.md
@@ -0,0 +1,8 @@
+---
+name: Contribution Guide
+description: Guidelines and instructions for submitting changes and generating commit messages.
+---
+
+# Contribution Guide
+
+When submitting changes or generating commit messages, you must strictly follow the formatting rules documented in [docs/commit_messages.md](./docs/commit_messages.md).