[Bug] SourceSnippetSanityTests anchors repo-root on CWD, not AppContext.BaseDirectory — can validate a sibling worktree

### What happened?

`tests/Reactor.DocPipeline.Tests/SourceSnippetSanityTests.cs:27` anchors its repo-root walk on the **process current directory**:

```csharp
private static string FindRepoRoot()
{
var dir = Directory.GetCurrentDirectory(); // <-- process CWD
while (dir is not null)
{
if (File.Exists(Path.Combine(dir, "Reactor.slnx")) || Directory.Exists(Path.Combine(dir, ".git")))
return dir;
dir = Path.GetDirectoryName(dir);
}
throw new InvalidOperationException("Reactor repo root not found from test cwd.");
}
```

Every other copy in the repo anchors on `AppContext.BaseDirectory` — the test assembly's own location, which is necessarily inside the tree that built it:

| file | anchor |
|---|---|
| `tests/Reactor.AppTests/Tests/Spec051DevtoolsPackageE2ETests.cs:131` | `AppContext.BaseDirectory` |
| `tests/Reactor.IntegrationTests/Packaging/CreateTemplateTests.cs:367` | `AppContext.BaseDirectory` |
| `tests/Reactor.DocPipeline.Tests/VersionSubstitutionTests.cs:213` | `AppContext.BaseDirectory` |
| `tests/Reactor.Tests/TemplateMetadataTests.cs:194` | `AppContext.BaseDirectory` |
| `tests/Reactor.Tests/VersionSingleSourceTests.cs:210` | `AppContext.BaseDirectory` |
| `src/Reactor.Cli/Pack/RepoRootFinder.cs:13` (the shared one) | `AppContext.BaseDirectory` |
| **`tests/Reactor.DocPipeline.Tests/SourceSnippetSanityTests.cs:27`** | **`Directory.GetCurrentDirectory()`** |

**Why it matters here specifically.** `AGENTS.md` tells contributors *"Work in a clean worktree, not `main`: `git worktree add …`"*, so a developer machine is expected to have **several valid checkouts of this repo at once**. A CWD-anchored walk finds whichever tree the process happens to be standing in. If that is a *sibling worktree*, `src/Reactor/Hooks/UseMemoCells.cs` exists there too, `ExtractFromSource` succeeds, and the test **passes green having validated a different tree than the one it was built from**. The repo's own recommended workflow is the precondition for the failure.

**Severity: low / latent**, and I'd rather state the mitigations than overclaim:

- The test runner normally sets CWD to the assembly output directory, so today this usually coincides with `AppContext.BaseDirectory`. I have not observed it fire.
- It `throw`s when no root is found, so the "wrong machine entirely" case fails **loudly** rather than silently.
- The silent-wrong-answer case needs CWD to sit inside *another checkout of this same repo* — real on a worktree-using dev box or a mutation/merge harness that drives a scratch worktree, but not on a clean CI runner.

The general hazard is worth recording regardless: `Set-Location`/`cd` in a driver script does **not** update the .NET process working directory, so managed relative-path resolution and shell relative-path resolution can silently disagree. Verified on this machine:

```
after Set-Location to :
Get-Location : C:\...\Temp\pA-4cefdf
[Environment]::CurrentDirectory : C:\...\
Set-Location updated CurrentDirectory? -> False
[IO.Path]::GetFullPath('probe.txt') : C:\...\\probe.txt <- not pA
```

- [x] I have searched existing issues and this isn't a duplicate.
- [x] This bug reproduces against the current `main` branch (the file is untouched by my branch).

Start at tests/Reactor.DocPipeline.Tests/SourceSnippetSanityTests.cs:27 and compare its FindRepoRoot anchor with AppContext.BaseDirectory and src/Reactor.Cli/Pack/RepoRootFinder.cs. Run the affected SourceSnippetSanityTests, including Extracts_demo_region_from_UseMemoCells, with separate worktrees; done means the test resolves files from the assembly's own checkout rather than the process CWD.

Written by the indexing model from the issue text.