refactor: move tasks into bundled plugin

docs/plugins/sdk-runtime.md

@@ -115,6 +115,40 @@ await api.runtime.subagent.deleteSession({
   Untrusted plugins can still run subagents, but override requests are rejected.
 </Warning>
 
+### `api.runtime.operations`
+
+Dispatch and query durable operation records behind a plugin-owned operations
+runtime.
+
+```typescript
+const created = await api.runtime.operations.dispatch({
+  type: "create",
+  namespace: "imports",
+  kind: "csv",
+  status: "queued",
+  description: "Import contacts.csv",
+  runId: "import-1",
+});
+
+const progressed = await api.runtime.operations.dispatch({
+  type: "transition",
+  runId: "import-1",
+  status: "running",
+  progressSummary: "Parsing rows",
+});
+
+const record = await api.runtime.operations.findByRunId("import-1");
+const list = await api.runtime.operations.list({ namespace: "imports" });
+const summary = await api.runtime.operations.summarize({ namespace: "imports" });
+```
+
+Notes:
+
+- `api.registerOperationsRuntime(...)` installs the active runtime.
+- Core exposes the facade; plugins own the operation semantics and storage.
+- The built-in default runtime maps the existing background task ledger into the
+  generic operations shape until a plugin overrides it.
+
 ### `api.runtime.tts`
 
 Text-to-speech synthesis.