fix: update to ACP SDK 2.0.0 (#1386)

Updates `@agentclientprotocol/sdk` from 1.9.0 to 2.0.0.

SDK 2.0.0 makes the default entrypoints stable-only and removes the
legacy connection classes ([migration
guide](https://github.com/agentclientprotocol/typescript-sdk/blob/main/MIGRATION_STABLE_UNSTABLE.md)).

- Protocol imports move to `@agentclientprotocol/sdk/unstable` and
`@agentclientprotocol/sdk/experimental/v2/unstable`. The agent uses
unstable features (session forking, `systemPrompt`, turn usage,
subagents, v2 draft extras), so this keeps the same protocol surface as
1.9.0. No wire changes.
- The scenario schema validator loads `schema/v1/schema.unstable.json`,
because the unversioned `schema/schema.json` export was removed.
- `remote-control.test.ts` uses the repo's own `AcpClient` type instead
of the removed `AgentSideConnection`.
- Runtime code already used the `agent()` app API, so it needed no
migration.

Verified: `npm run build`, `npm run test:run` (3199 passed), `npm run
check`, and a separate typecheck of `examples/`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

examples/simple-client.ts

@@ -82,10 +82,10 @@ import {
   client as acpClient,
   methods,
   ndJsonStream,
-} from "@agentclientprotocol/sdk";
+} from "@agentclientprotocol/sdk/unstable";
 // Type-only: Node's type stripping does not elide value imports, and these two
 // exist only in the .d.ts — importing them normally is a runtime SyntaxError.
-import type { AnyMessage, Stream } from "@agentclientprotocol/sdk";
+import type { AnyMessage, Stream } from "@agentclientprotocol/sdk/unstable";
 
 // ---------------------------------------------------------------- steering
 

examples/steering.ts

@@ -46,7 +46,7 @@ import {
   ndJsonStream,
   type PromptRequest,
   type PromptResponse,
-} from "@agentclientprotocol/sdk";
+} from "@agentclientprotocol/sdk/unstable";
 
 /** The steering extension method, per the ACP steering wire protocol. */
 const STEERING_METHOD = "_session/steering";