PowerShell hook crashes with empty $PLUGIN_ROOT when a non-primary workspace folder's .github/hooks/*.json is invoked in a multi-root workspace

### Summary
In a multi-root workspace (Editor Window), when an Agent Host session is anchored to folder B's working directory but folder A also defines a repository-level hook (`A/.github/hooks/*.json`), VS Code still tries to invoke A's hook — consistent with the aggregation-across-all-folders behavior described in #328021 / verified in #331413. On Windows, invoking that hook fails outright rather than silently no-op'ing, because the generated PowerShell wrapper resolves the hook script path via `Join-Path "${PLUGIN_ROOT}" 'hooks/...'`, and `PLUGIN_ROOT` is empty for a hook config that doesn't belong to the session's own working directory:

```
Join-Path : Cannot bind argument to parameter 'Path' because it is an empty string.
At line:1 char:62
+ ... issionDecision":"allow"}'; $hook = Join-Path "${PLUGIN_ROOT}" 'hooks/ ...
+ ~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidData: (:) [Join-Path], ParameterBindingValidationException
+ FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,Microsoft.PowerShell.Commands.JoinPathCommand
```

### Steps to reproduce
1. Open a multi-root workspace containing folder A (with a `A/.github/hooks/*.json` repository-level `postToolUse` command hook using the `powershell` field) and folder B (no hooks of its own).
2. Start/resume an Agent Host session anchored to folder B's working directory.
3. Trigger a tool call that would match A's hook (e.g. an `edit`/`create` tool use).
4. Observe the PowerShell `Join-Path` exception surfaced in the session instead of the hook being skipped or resolved correctly.

### Expected
Either: the session should not receive A's hook at all (per the scoping fix in #328021), or if cross-folder hooks are intentionally still invoked, `PLUGIN_ROOT` should resolve to A's folder path rather than being empty, so the hook runs (or fails) the same way it would from A's own session.

### Environment
- Windows, VS Code multi-root workspace (`.code-workspace` with 3 folders)
- Repository-level hook defined via `.github/hooks/*.json` with a `powershell` command entry
- Related: #328021, #331413

### Impact
Every agent task run against folder B/C fails/logs spurious errors due to a hook that was only ever meant to apply to folder A.