fix: make plugin hashes reproducible so the fork-PR verify gate can pass

[Hirogen]

The Verify Plugin Hashes (fork PRs) step in build_dotnet.yml regenerates PluginHashGenerator.Generated.cs and runs git diff --exit-code on it. That gate could never pass, for two independent reasons.

First, the generator embedded DateTime.UtcNow in the file, so every regeneration produced a diff even when no hash had changed.

Second, the SDK queries git and feeds the commit sha into every assembly by two routes: appended to InformationalVersion, and into the SourceLink map in the portable PDB, whose checksum is embedded in the DLL. So the hashes you commit describe the parent commit's binaries and go stale the instant they land - committing them changes HEAD, which changes the binaries again. Only first-party assemblies moved; the NuGet-supplied DLLs alongside them never did.

That made the gate unwinnable rather than merely awkward: PR #673 burned two attempts re-pasting hashes, and every push produced a different set again.

On Development the same churn was absorbed by the sibling auto-commit step, which is why chore: update plugin hashes [skip ci] landed after nearly every merge. It also meant the committed table always described a different build than the one it shipped in, so Release plugin verification never matched its own binaries.

Drop the timestamp, stop the SDK's source-control queries, and regenerate. Verified: two clean Release builds at different commits now produce identical plugin hashes, and git diff --exit-code on the generated file is clean.