Add python_git: a leak-safe GitPython alternative

Every git invocation runs through a single blocking subprocess.run
call in python_git/runner.py that always waits for the process to
exit and closes its pipes before returning, so there is no long-lived
Popen handle for a caller to forget to close (the root cause of
GitPython's known fd/process leaks).

Repo exposes status/log/diff/add/commit/push/pull/fetch, branch and
remote management, and clone, backed by dataclasses in models.py.
Covered by tests/test_repo.py, including an fd-count regression test.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/

.idea/inspectionProfiles/profiles_settings.xml

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+<component name="InspectionProjectProfileManager">
+  <settings>
+    <option name="USE_PROJECT_PROFILE" value="false" />
+    <version value="1.0" />
+  </settings>
+</component>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" project-jdk-name="~/PycharmProjects/python-git/.venv" project-jdk-type="Python SDK" />
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/python-git.iml" filepath="$PROJECT_DIR$/.idea/python-git.iml" />
+    </modules>
+  </component>
+</project>

.idea/python-git.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module external.system.id="pyproject.toml" type="PYTHON_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$">
+      <excludeFolder url="file://$MODULE_DIR$/.venv" />
+    </content>
+    <orderEntry type="jdk" jdkName="~/PycharmProjects/python-git/.venv" jdkType="Python SDK" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+  <component name="PackageRequirementsSettings" />
+  <component name="PyDocumentationSettings" />
+  <component name="ReSTService" />
+  <component name="TestRunnerService" />
+</module>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>