feat(agents): add gitIdentity config and git-identity rule

- coding.agents.gitIdentity enabled with m3ta-chiron identity
- coding.agents.pi.codingRules.concerns includes 'git-identity'
- Uses feature/agent-git-identity branches for m3ta-nixpkgs and agents

home/coding/agents/pi.nix

@@ -23,6 +23,7 @@
         "documentation" # Documentation standards
         "testing" # Testing guidelines (Arrange-Act-Assert)
         "git-workflow" # Conventional commits, branch naming
+        "git-identity" # Git identity configuration for agents
         "project-structure" # Project layout conventions
       ];