Detect external changes with deep context (#1888)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Adds commit-aware deep context by computing hasExternalChanges (via
latest assistant commit vs current repo + dirty check) and propagating
commitHash through messages/provider options.
> 
> - **Deep Smart Context**:
> - Add `hasExternalChanges` to `VersionedFiles`; compute by comparing
latest assistant `commitHash` with `getCurrentCommitHash` and checking
`isGitStatusClean`.
> - Make `sourceCommitHash` nullable; add `commitHash` in
`DyadEngineProviderOptions` and use it when scanning history.
> - **Chat Handling**:
> - Include `commitHash` in `messageHistory` and pass through
`providerOptions['dyad-engine']`.
> - **Git Utilities**:
> - New `isGitStatusClean(path)` supporting native git and
isomorphic-git.
> - **Tests/Snapshots**:
> - Mock `getCurrentCommitHash` and `isGitStatusClean`; update snapshot
to include `hasExternalChanges`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ad92d9dd5ead941de822e8da59c8819e4db8b775. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->







<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Detects external code changes in deep context by comparing the latest
assistant commit with the current repo state. Exposes a
hasExternalChanges flag so the engine can adapt responses when the
workspace diverges.

- **New Features**
  - Added hasExternalChanges to VersionedFiles.
- Computes by comparing the latest assistant commitHash with
getCurrentCommitHash and checking isGitStatusClean.
- Passes commitHash through chat messages and dyad-engine
providerOptions; sourceCommitHash is now nullable.
  - Defaults to true if detection fails (with a warning).

<sup>Written for commit 6ebb0b125c9a3421b4e5673870b204c9cb279265.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

e2e-tests/snapshots/smart_context_deep.spec.ts_smart-context-deep---read-write-read-1.txt

@@ -525,7 +525,8 @@
             "src/pages/Index.tsx": "63e4529ce654d0667a5204e3ec99ea29aac4ad8967682eb9158506beda92b4eb"
           },
           "7": {}
-        }
+        },
+        "hasExternalChanges": false
       },
       "enable_lazy_edits": true,
       "enable_smart_files_context": true,