feat(KB-203): add git commands to CLI

- Add git command module with status, push, pull, and fetch subcommands

- Integrate git commands into CLI routing and help system

- Add comprehensive unit tests for all git commands (657 lines)

- Include changeset for the new CLI git operations

.changeset/add-cli-git-commands.md

@@ -0,0 +1,6 @@
+---
+"@dustinbyrne/kb": minor
+---
+
+Add `kb git` subcommand group with status, push, pull, and fetch commands
+for convenient git workflows from the CLI.