build: derive package version from git via setuptools-scm

Replace hatchling and the static version with setuptools + setuptools-scm,
following Kaya's pre-uv packaging setup: dynamic version, generated
_version.py, and release/* tag prefix. The Docker builder now includes git
and the .git directory so the image is versioned from the checkout. CI
checks out full history and tags for the tag-triggered builds.

.dockerignore

@@ -1,4 +1,3 @@
-.git/
 .gitignore
 .venv/
 __pycache__/
@@ -15,3 +14,8 @@ Dockerfile
 !README.md
 xmcp.yml
 xmcp.example.yml
+
+# Generated at build time
+src/xmcp/_version.py
+*.egg-info/
+dist/