feat: turn session recording into a plugin (B13)

packages/plugin-sdk/src/backend.ts

@@ -74,6 +74,19 @@ export interface PluginKeyValue {
   list: () => Promise<string[]>;
 }
 
+/**
+ * A folder on disk this plugin owns, for state too large for ctx.kv (session
+ * recordings, uploaded files). Requires files:own.
+ */
+export interface PluginFiles {
+  /**
+   * The plugin's own folder under DATA_DIR, created if it does not exist yet.
+   * Always the same path for this plugin; a plugin lays out its own
+   * subdirectories underneath it.
+   */
+  dataDir: () => Promise<string>;
+}
+
 /**
  * The plugin's own tables.
  *
@@ -1147,6 +1160,7 @@ export interface PluginContext {
   readonly log: PluginLogger;
   readonly events: PluginEvents;
   readonly kv: PluginKeyValue;
+  readonly files: PluginFiles;
   readonly db: PluginDatabase;
   readonly sync: PluginSync;
   readonly registry: PluginRegistry;

packages/plugin-sdk/src/capabilities.ts

@@ -60,6 +60,7 @@ export const CAPABILITY_CATALOG: readonly CapabilityInfo[] = [
   entry("hosts:read", "low"),
   entry("db:own", "low"),
   entry("kv:own", "low"),
+  entry("files:own", "low"),
   entry("secrets:own", "low"),
   entry("settings:read-core", "low"),
   entry("ui:surface", "low"),

packages/plugin-sdk/src/testing.ts

@@ -300,6 +300,12 @@ export function createFakeContext(
       list: async () => [...kv.keys()],
     },
 
+    files: {
+      // A fake path; a test that needs a real folder uses the OS temp dir
+      // itself, not this double.
+      dataDir: async () => `/tmp/${pluginId}`,
+    },
+
     db: options.db
       ? {
           ...options.db,
@@ -798,6 +804,13 @@ export function createMockCtx(
       },
     },
 
+    files: {
+      dataDir: async () => {
+        require("files:own");
+        return ctx.files.dataDir();
+      },
+    },
+
     events: {
       emit: (topic, payload) => {
         if (