kleaf: add creating-kernel-commits skill and evals

Introduce the creating-kernel-commits skill to guide AI coding
assistants when drafting commit messages in Android kernel and tooling
repositories.

The skill outlines general format rules (summary, body, trailers
sections), project-specific guidelines (ACK, Kleaf, build-tools,
external), and AI-specific rules (no sign-offs, mandatory Assisted-by).
Also include the EVAL.yaml file with evaluation test cases to verify
the skill.

TAG=agy
CONV=15751285-e0e5-4948-8b57-6a792ed63561

Bug: 507499871
Test: From <workspace_root>/build/kernel: run_evals.py --skill_dir .agents/skills/creating_kernel_commits
Assisted-by: Antigravity:Gemini-Next
Change-Id: I931800842f8c1568dd83940137102bcb17cde28d