GetDefaultRepoRoot() throws in git worktrees when running dotnet run --project

[BenjaminMichaelis]

Description

RepositoryPaths.GetDefaultRepoRoot() throws InvalidOperationException: SolutionDir is not a valid directory when tests
are run via dotnet run --project from a git worktree.

Two independent failure modes combine to produce this:

1. $(SolutionDir) is unset during project-level builds. The WriteLaunchers MSBuild target (in
   IntelliTect.Multitool.targets) writes SolutionDir to %TEMP%\IntelliTect.MultiTool.BuildVariables.tmp at build time. When
   building via dotnet run --project, MSBuild has no solution context, so $(SolutionDir) is an empty string — and that
   gets written to the temp file, invalidating any previously correct value.
2. The .git-walk fallback fails in git worktrees. TrySearchForGitContainingDirectory walks up the directory tree
   looking for a .git directory. In a git worktree, the checkout contains a .git file (a gitfile pointer) instead of a
   directory. The walk finds the file but not a directory, so the fallback also fails.

Either fix alone would resolve the issue; both need to be addressed for full robustness.

---

Repro Steps

1. Clone the repo and create a worktree: git clone https://github.com//.git main
   cd main
   git worktree add ../my-branch my-branch
   cd ../my-branch
2. Add a dependency on IntelliTect.Multitool in a test project that calls RepositoryPaths.GetDefaultRepoRoot() at
   runtime.
3. Run the tests directly against the project (not the solution): dotnet run --project MyProject.Tests
4. Observe the exception: InvalidOperationException: SolutionDir is not a valid directory.
   at IntelliTect.Multitool.RepositoryPaths.GetDefaultRepoRoot()

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Workaround

Build from the solution first (which writes a valid SolutionDir to the temp file), then run with --no-build to skip the
project-level rebuild that would overwrite it:

dotnet build MyApp.sln
dotnet run --project MyProject.Tests --no-build

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Expected Behavior

GetDefaultRepoRoot() should successfully locate the repo root from a git worktree, either by:

• Recognising a .git file (not just a .git directory) as a valid repo root marker in
  TrySearchForGitContainingDirectory, and/or
• Not overwriting the temp file with an empty SolutionDir when no solution is in context (e.g. only write when
  $(SolutionDir) is non-empty)

---

Environment

• IntelliTect.Multitool version: 1.5.3
• Git feature: worktrees (git worktree add)
• Build tool: dotnet run --project (no solution file in context)