Isolate Git-based tests from user configuration

[Widthdom]

CLOSES #217

Summary

• add a shared TestGitRepository initialization path for every temporary Git repository used by the test suite
• isolate global/system Git configuration, user home/XDG files, attributes, templates, hooks, repository/identity environment variables, and interactive prompts
• set deterministic local identity and explicitly disable commit/tag signing without modifying user configuration
• migrate the three release tag-resolution tests to the shared helper
• add regression coverage for global commit/tag signing, missing signing keys, default user excludes, and annotated tags
• document the common setup in the bilingual testing guide and changelog

Root cause and impact

The existing tests initialized repositories directly and therefore inherited settings such as commit.gpgSign=true. Signed commits could prompt, fail, or hang on developer machines. Other inherited inputs such as GIT_CONFIG and per-user ignore files could also make otherwise deterministic test setup fail.

Validation

• dotnet build FolderDiffIL4DotNet.sln --configuration Release --no-restore --nologo -m:1
• dotnet test FolderDiffIL4DotNet.Tests/FolderDiffIL4DotNet.Tests.csproj --configuration Release --no-restore --nologo -m:1 — 2035 passed, 1 existing E2E opt-in skip
• focused regression run with parent GIT_CONFIG=/dev/null — 4 passed
• changed C# files pass dotnet format --verify-no-changes
• python3 scripts/validate-test-scope-map.py — 93/93 test classes documented
• git diff --check
• adversarial review of origin/main..HEAD — no remaining findings

The repository-wide format check still reports pre-existing whitespace findings in unrelated files; all C# files changed by this PR pass the scoped format check.