🔒️(ci) install every Python job through uv

lint-git runs gitlint through uvx, which fetches it for that one command and
puts nothing in the runner's Python. lint-summary and test-summary sync from a
new src/summary/uv.lock, so the versions those jobs resolve are the versions in
the tree, which is what src/backend and src/agents already do. The agents sync
refuses source distributions, since that package installs no project of its own.

Follows suitenumerique/menshen#73.

src/summary/pyproject.toml

@@ -2,6 +2,7 @@
 [project]
 name = "summary"
 version = "1.29.0"
+requires-python = ">=3.13"
 dependencies = [
     "fastapi[standard]>=0.105.0",
     "uvicorn>=0.24.0",