feat: upgrade to upstream v2026.3.28

Major upgrade from e26988a38 to upstream v2026.3.28 (f9b107928).
Key changes:
- Upstream src/, ui/, extensions/ (89 bundled extensions)
- Zero-token web providers preserved in src/zero-token/
- AskOnce plugin restored and registered as CLI command
- Added missing packages: @anthropic-ai/vertex-sdk, @modelcontextprotocol/sdk
- Fixed tsconfig rootDir, skipLibCheck for plugin-sdk DTS build
- Added askonce to bundled plugin metadata and package.json exports
- Fixed AskOnce CLI command registration (missing commands metadata)
- Restored AskOnce adapter imports (correct 5-level relative paths)
- Removed stale migration artifacts from root directory

cli/program/command-tree.test.ts

@@ -0,0 +1,39 @@
+import { Command } from "commander";
+import { describe, expect, it } from "vitest";
+import { removeCommand, removeCommandByName } from "./command-tree.js";
+
+describe("command-tree", () => {
+  it("removes a command instance when present", () => {
+    const program = new Command();
+    const alpha = program.command("alpha");
+    program.command("beta");
+
+    expect(removeCommand(program, alpha)).toBe(true);
+    expect(program.commands.map((command) => command.name())).toEqual(["beta"]);
+  });
+
+  it("returns false when command instance is already absent", () => {
+    const program = new Command();
+    program.command("alpha");
+    const detached = new Command("beta");
+
+    expect(removeCommand(program, detached)).toBe(false);
+  });
+
+  it("removes by command name", () => {
+    const program = new Command();
+    program.command("alpha");
+    program.command("beta");
+
+    expect(removeCommandByName(program, "alpha")).toBe(true);
+    expect(program.commands.map((command) => command.name())).toEqual(["beta"]);
+  });
+
+  it("returns false when name does not exist", () => {
+    const program = new Command();
+    program.command("alpha");
+
+    expect(removeCommandByName(program, "missing")).toBe(false);
+    expect(program.commands.map((command) => command.name())).toEqual(["alpha"]);
+  });
+});