fix: add dispatch_context to schema docs and test buildDispatchMetadata

- Add dispatch_context column to requirements.md task schema section
- Export buildDispatchMetadata for test coverage
- Add tests: context forwarding, task_uuid overwrite protection, empty context

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/ai_client/client.go

@@ -196,6 +196,11 @@ func (d *Dispatcher) buildHistory(threadID string) ([]historyEntry, error) {
 	return history, nil
 }
 
+// ExportBuildDispatchMetadata exports buildDispatchMetadata for testing.
+func ExportBuildDispatchMetadata(task *database.Task) map[string]string {
+	return buildDispatchMetadata(task)
+}
+
 // buildDispatchMetadata constructs the metadata map for OpenClaw dispatch,
 // merging the task UUID with any DispatchContext from the rules pipeline.
 func buildDispatchMetadata(task *database.Task) map[string]string {