feat(ssh): wave 5a — exec-backend remote-backend factory handle

exec-backend declares remoteExecBackendFactoryHandle (a consumer-defined
ServiceHandle<(computerId) => ExecBackend>) that the ssh package will provide
(standard→core layering). The resolver's computerId-set branch now lazy-looks-up
this factory (at tool-execute time, runtime) and calls it; if ssh isn't loaded,
getService throws → a clear 'SSH remote execution is not configured' error. The
computerId-undefined (local) branch is byte-identical to before.

This is the seam wave 5b (the ssh package) plugs into. +tests for both branches.

Verified: tsc -b EXIT 0, biome clean. No merge or push.

1 files changed, 4 insertions, 1 deletions

diff --git a/packages/exec-backend/src/index.ts b/packages/exec-backend/src/index.ts
index 3135f79..30c12c8 100644
--- a/packages/exec-backend/src/index.ts
+++ b/packages/exec-backend/src/index.ts
@@ -2,4 +2,7 @@ export type { DirEntry, ExecBackend, ExecResult, SpawnParams, StatResult } from
 export { createExecBackendExtension, manifest } from "./extension.js";
 export { createLocalExecBackend, localExecBackend } from "./local.js";
 export type { ExecBackendResolver } from "./service.js";
-export { execBackendHandle } from "./service.js";
+export {
+	execBackendHandle,
+	remoteExecBackendFactoryHandle,
+} from "./service.js";