Revert "refactor: move tasks into bundled plugin"

This reverts commit c75f4695b7.

docs/plugins/sdk-runtime.md

@@ -115,40 +115,6 @@ 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.