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/mod.rs

@@ -0,0 +1,39 @@
+//! SDK plumbing — types and the per-call registration entry point.
+//!
+//! `executor-core` is responsible for building the per-invocation Rhai
+//! engine and wiring stateful services into it. v1.1.0 ships the
+//! shapes (`Services` bundle, `SdkCallCx`, `register_all` entry point)
+//! but no actual services — subsequent v1.1.x PRs (KV in v1.1.1,
+//! docs in v1.1.2, …) extend `register_all` rather than re-threading
+//! plumbing through `engine.rs`.
+//!
+//! Bridge functions (`json_to_dynamic` / `dynamic_to_json`) also live
+//! here so service modules can convert values without `engine.rs`
+//! being the only home for the conversion logic.
+
+pub mod bridge;
+pub mod cx;
+
+pub use bridge::{dynamic_to_json, json_to_dynamic};
+pub use cx::SdkCallCx;
+
+use std::sync::Arc;
+
+use picloud_shared::Services;
+use rhai::Engine as RhaiEngine;
+
+/// Single hook every v1.1.x stateful service registers into. Called
+/// once per invocation, just after `build_engine` constructs the
+/// sandboxed Rhai engine and just before script compilation.
+///
+/// v1.1.0 ships an intentionally empty body — the call site exists so
+/// future PRs (KV first) drop their registration logic here rather
+/// than reaching into `engine.rs::build_engine`. The signature is
+/// locked: subsequent PRs MUST keep the same parameter shape so that
+/// hosts don't have to re-thread the plumbing.
+pub fn register_all(engine: &mut RhaiEngine, services: &Services, cx: Arc<SdkCallCx>) {
+    // Intentionally inert in v1.1.0. The unused-suppression below is a
+    // load-bearing placeholder: future PRs replace this `let _` with
+    // real `register_kv(engine, services, cx.clone())` calls etc.
+    let _ = (engine, services, cx);
+}