refactor(executor-core): extract sdk/ module + move json↔dynamic bridge

Hoist the json_to_dynamic / dynamic_to_json helpers out of engine.rs
into a new sdk/bridge.rs so the v1.1.x service modules (KV, docs, …)
can use them without engine.rs being the sole owner. No behavioural
change — the sdk_contract round-trip test pins the observable JSON
fidelity.

Also lands the structural shape that subsequent v1.1.x PRs hook into:

  - sdk::register_all(engine, services, cx) — single per-call hook
    every stateful service registers through. Body is a no-op for
    v1.1.0; SdkCallCx construction inside Engine::execute lands in
    the next commit alongside the new ExecRequest fields it reads.
  - sdk::cx re-exports picloud_shared::SdkCallCx so SDK callers don't
    cross-import shared for one type.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/executor-core/src/lib.rs

@@ -8,6 +8,7 @@ pub mod context;
 pub mod engine;
 pub mod logging;
 pub mod sandbox;
+pub mod sdk;
 pub mod types;
 
 pub use engine::Engine;