Add schema get command to CLI and HTTP API

Exposes the existing schema_source() method via a new `omnigraph schema get`
CLI subcommand and a `GET /schema` API endpoint, allowing users to retrieve
the current accepted schema from any graph repository.

https://claude.ai/code/session_01UYybeBQks3fz3RJrTHtwQw

crates/omnigraph-server/src/api.rs

@@ -280,6 +280,11 @@ pub struct SchemaApplyOutput {
     pub steps: Vec<SchemaMigrationStep>,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
+pub struct SchemaGetOutput {
+    pub source: String,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
 pub struct IngestRequest {
     pub branch: Option<String>,