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/sdk/cx.rs

@@ -0,0 +1,10 @@
+//! Re-export of `picloud_shared::SdkCallCx`.
+//!
+//! The type itself lives in `picloud-shared` because future stateful
+//! service impls live in `manager-core` (which `executor-core` must
+//! not depend on) and need to reference the same cx shape. This
+//! re-export lets executor-side code write
+//! `use picloud_executor_core::sdk::SdkCallCx;` instead of reaching
+//! into `picloud_shared` for one type.
+
+pub use picloud_shared::SdkCallCx;