feat(security): add a user-writable plugin ingest inbox for cross-account data

The LocalService runner can't traverse the interactive user's profile the way
the old SYSTEM runner could — so a plugin can no longer read a file an app
running as *you* produced (the acute case: the Playnite exporter's library
JSON under %APPDATA%). Confirmed on-glass: as LocalService the glob finds
nothing and the profile is un-traversable.

Add the inverse of plugin-state: <config_dir>\ingest, granted BUILTIN\Users
Modify by plugins enable (disable reverts to inherited Users:RX). An
interactive-user app drops ingest\<plugin>\… and the de-privileged runner
reads it there — the one Users-writable carve-out in the otherwise
Users-read-only tree. SDK exports pluginIngestDir(name) to resolve it; on
Linux the systemd --user runner owns the config dir so same-user producers
write there with no grant.

Accepted tradeoff: the inbox is writable by any local user (trusted-single-
user model; it feeds only a LocalService runner). Consumers must treat ingest
data as lower trust than their own state.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sdk/test/config.test.ts

@@ -2,7 +2,7 @@
 // plugin persists into — the one dir the de-privileged Windows runner may write.
 import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import * as path from "node:path";
-import { pluginStateDir } from "../src/config.js";
+import { pluginIngestDir, pluginStateDir } from "../src/config.js";
 
 describe("pluginStateDir", () => {
 	let saved: string | undefined;
@@ -27,3 +27,24 @@ describe("pluginStateDir", () => {
 		expect(pluginStateDir("x").startsWith(pluginStateDir())).toBe(true);
 	});
 });
+
+describe("pluginIngestDir", () => {
+	let saved: string | undefined;
+	beforeEach(() => {
+		saved = process.env.PUNKTFUNK_CONFIG_DIR;
+	});
+	afterEach(() => {
+		if (saved === undefined) delete process.env.PUNKTFUNK_CONFIG_DIR;
+		else process.env.PUNKTFUNK_CONFIG_DIR = saved;
+	});
+
+	test("resolves <config_dir>/ingest[/name], distinct from plugin-state", () => {
+		process.env.PUNKTFUNK_CONFIG_DIR = path.join("/tmp", "pf-cfg3");
+		expect(pluginIngestDir()).toBe(path.join("/tmp", "pf-cfg3", "ingest"));
+		expect(pluginIngestDir("playnite")).toBe(
+			path.join("/tmp", "pf-cfg3", "ingest", "playnite"),
+		);
+		// the inbox (Users-write) is a different tree from state (LocalService-write)
+		expect(pluginIngestDir("playnite")).not.toBe(pluginStateDir("playnite"));
+	});
+});