Bound subprocess execution for git helpers and upgrade installer

[Widthdom]

Summary

• Bound GitHelper subprocess runtime and captured stdout/stderr, returning explicit diagnostics when timeout or capture limits are hit.
• Launch cdidx upgrade installer with ProcessStartInfo.ArgumentList and terminate stalled installer runs with a clear error.
• Added regression coverage for git timeout/output caps and upgrade installer argument/timeout behavior.

Validation

• dotnet build -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~GitHelperTests -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~ProgramRunnerTests -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• Adversarial review: no blocking/actionable issues remain after fixes. External codex exec review --base origin/main --ephemeral was attempted twice but blocked by Codex usage/model availability limits in this environment.

Documentation / Changelog

• Added changelog.d/unreleased/2832.fixed.md.
• Added changelog.d/unreleased/2827.fixed.md.
• No direct CHANGELOG.md update; ordinary issue-fix fragments were used.

Follow-up Candidates

• None.

Fixes #2832
Fixes #2827