[Repo Assist] fix: read phpcbf.onsave per-document URI in onWillSaveTextDocument

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🤖 This is an automated draft PR from Repo Assist, an AI assistant.

Summary

Fixes a multi-root workspace bug where phpcbf.onsave was read from the shared PHPCBF instance rather than scoped to the document being saved. This caused incorrect behaviour when different workspace folders had different phpcbf.onsave values.

Root Cause

phpcbf.onsave is a resource-scoped setting ("scope": "resource" in package.json), meaning it can differ per workspace folder in a multi-root setup. However, the onWillSaveTextDocument handler was checking phpcbf.onsave — a property on the shared instance that reflects whatever loadSettings() was last called with (the constructor, or an onDidChangeConfiguration event, neither of which are per-document).

Reproduction (multi-root workspace)

1. Open two workspace folders: folder A and folder B
2. Folder A settings: "phpcbf.onsave": false
3. Folder B settings: "phpcbf.onsave": true
4. Activate a file from folder B (this populates phpcbf.onsave = true in the instance)
5. Save a PHP file from folder A — formatting fires incorrectly, because phpcbf.onsave is still true

The reverse is also true: save a file from folder B after activating a file from folder A, and formatting does not fire when it should.

Fix

Read phpcbf.onsave directly from workspace.getConfiguration("phpcbf", event.document.uri) in the handler — identical to how editor.formatOnSave is already read on the very next line. This is the correct per-resource scoping pattern for VS Code extensions.

Before / After

// Before — uses shared instance property (wrong scope in multi-root)
if (event.document.languageId == "php" && phpcbf.onsave && ...)

// After — reads setting scoped to the document's URI (correct)
const onsave = workspace.getConfiguration("phpcbf", event.document.uri).get("onsave", false);
if (event.document.languageId == "php" && onsave && ...)
```

## Trade-offs / Notes

- This is a minimal, self-contained change — one line replaced with three
- Does not conflict with any other in-progress functionality
- The fix is consistent with how `editor.formatOnSave` is already read scoped to the document
- PR #53 also modifies the `onWillSaveTextDocument` handler — if both are merged, the per-resource onsave read should be preserved

## Test Status

All 7 unit tests pass (`npm run test:unit`). The extension tests require the VS Code host environment and cannot run in CI.

```
✔ findFiles (7 tests)

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