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.

.gitignore

@@ -10,3 +10,9 @@ xmcp.yml
 xmcp.local.yml
 .env
 .env.*
+
+# Generated version metadata and build artifacts
+src/xmcp/_version.py
+*.egg-info/
+build/
+dist/