refactor(git): introduce GitRunError hierarchy, surface non-repo errors, and improve verification queries

* Replace legacy GitError usage with a clearer exception hierarchy:

  * GitBaseError as the common root for all git-related failures
  * GitRunError for subprocess execution failures
  * GitQueryError for read-only query failures
  * GitCommandError for state-changing command failures
  * GitNotRepositoryError to explicitly signal “not a git repository” situations
* Update git runner to detect “not a git repository” stderr and raise GitNotRepositoryError with rich context (cwd, command, stderr)
* Refactor repository verification to use dedicated query helpers instead of ad-hoc subprocess calls:

  * get_remote_head_commit (ls-remote) for pull mode
  * get_head_commit for local mode
  * get_latest_signing_key (%GK) for signature verification
* Add strict vs best-effort behavior in verify_repository:

  * Best-effort collection for reporting (does not block when no verification config exists)
  * Strict retrieval and explicit error messages when verification is configured
  * Clear failure cases when commit/signing key cannot be determined
* Add new unit tests covering:

  * get_latest_signing_key output stripping and error wrapping
  * get_remote_head_commit parsing, empty output, and error wrapping
  * verify_repository success/failure scenarios and “do not swallow GitNotRepositoryError”
* Adjust imports and exception handling across actions/commands/queries to align with GitRunError-based handling while keeping GitNotRepositoryError uncaught for debugging clarity

https://chatgpt.com/share/6943173c-508c-800f-8879-af75d131c79b

tests/e2e/test_changelog_commands.py

@@ -89,7 +89,7 @@ class TestIntegrationChangelogCommands(unittest.TestCase):
         """
         Run 'pkgmgr changelog HEAD~5..HEAD' inside the pkgmgr repo.
         Selbst wenn HEAD~5 nicht existiert, sollte der Befehl den
-        GitError intern behandeln und mit Exit-Code 0 beenden
+        GitBaseError intern behandeln und mit Exit-Code 0 beenden
         (es wird dann eine [ERROR]-Zeile gedruckt).
 
         Wird übersprungen, wenn das pkgmgr-Repo nicht lokal vorhanden ist.