docs(plugins): document context engine slots and registration

docs/tools/plugin.md

@@ -393,13 +393,29 @@ Some plugin categories are **exclusive** (only one active at a time). Use
   plugins: {
     slots: {
       memory: "memory-core", // or "none" to disable memory plugins
+      contextEngine: "legacy", // or a plugin id such as "lossless-claw"
     },
   },
 }
 ```
 
-If multiple plugins declare `kind: "memory"`, only the selected one loads. Others
-are disabled with diagnostics.
+Supported exclusive slots:
+
+- `memory`: active memory plugin (`"none"` disables memory plugins)
+- `contextEngine`: active context engine plugin (`"legacy"` is the built-in default)
+
+If multiple plugins declare `kind: "memory"` or `kind: "context-engine"`, only
+the selected plugin loads for that slot. Others are disabled with diagnostics.
+
+### Context engine plugins
+
+Context engine plugins own session context orchestration for ingest, assembly,
+and compaction. Register them from your plugin with
+`api.registerContextEngine(id, factory)`, then select the active engine with
+`plugins.slots.contextEngine`.
+
+Use this when your plugin needs to replace or extend the default context
+pipeline rather than just add memory search or hooks.
 
 ## Control UI (schema + labels)
 
@@ -465,6 +481,37 @@ Plugins export either:
 - A function: `(api) => { ... }`
 - An object: `{ id, name, configSchema, register(api) { ... } }`
 
+Context engine plugins can also register a runtime-owned context manager:
+
+```ts
+export default function (api) {
+  api.registerContextEngine("lossless-claw", () => ({
+    info: { id: "lossless-claw", name: "Lossless Claw", ownsCompaction: true },
+    async ingest() {
+      return { ingested: true };
+    },
+    async assemble({ messages }) {
+      return { messages, estimatedTokens: 0 };
+    },
+    async compact() {
+      return { ok: true, compacted: false };
+    },
+  }));
+}
+```
+
+Then enable it in config:
+
+```json5
+{
+  plugins: {
+    slots: {
+      contextEngine: "lossless-claw",
+    },
+  },
+}
+```
+
 ## Plugin hooks
 
 Plugins can register hooks at runtime. This lets a plugin bundle event-driven