chore: initial scaffold — workspace, docs, blueprint

Sets up the PiCloud monorepo as a Cargo workspace organised around the
three-service architecture (manager / orchestrator / executor), each
backed by a *-core library crate so the same logic powers both the MVP
all-in-one `picloud` binary and the future split-process cluster mode.

  * crates/shared, executor-core, orchestrator-core, manager-core
    define the library surface and trait seams between the three
    services (`ExecutorClient`, `ScriptResolver`, `ScriptRepository`).
  * crates/picloud is the MVP entrypoint; serves /healthz on 8080
    (override via PICLOUD_BIND).
  * crates/picloud-{manager,orchestrator,executor} are skeleton
    binaries that keep the crate boundaries honest until cluster
    mode is built out in v1.3+.
  * docs/git-workflow.md defines the trunk-based workflow:
    short-lived branches, Conventional Commits, separate hotfix
    flow with mandatory reproduction tests.
  * CLAUDE.md captures the working rules for future Claude sessions.

Workspace passes `cargo fmt`, `cargo clippy -D warnings` (with
pedantic enabled), and `cargo test --workspace`. The all-in-one
binary responds on `/healthz` and `/`.

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

crates/executor-core/src/engine.rs

@@ -0,0 +1,48 @@
+use crate::sandbox::Limits;
+use crate::types::{ExecError, ExecRequest, ExecResponse};
+
+/// Preconfigured Rhai engine with sandbox limits applied.
+///
+/// One `Engine` is constructed at process startup and reused across
+/// invocations. `execute` is the only entry point — it owns the per-call
+/// scope and log buffer, then returns a complete `ExecResponse`.
+pub struct Engine {
+    limits: Limits,
+    // The actual `rhai::Engine` lands with the first execution implementation.
+    // Keep this opaque for now so callers don't bind to it.
+}
+
+impl Engine {
+    #[must_use]
+    pub fn new(limits: Limits) -> Self {
+        Self { limits }
+    }
+
+    #[must_use]
+    pub fn limits(&self) -> &Limits {
+        &self.limits
+    }
+
+    /// Parse-only validation, used by the manager at script-upload time
+    /// so syntax errors are surfaced before the first invocation.
+    pub fn validate(&self, _source: &str) -> Result<(), ExecError> {
+        // TODO(executor-core): wire `rhai::Engine::compile`
+        Ok(())
+    }
+
+    /// Execute `source` against `req` under the configured sandbox.
+    ///
+    /// `async` is part of the contract: v1.1+ SDK calls (kv, docs, http)
+    /// will await injected service providers from inside this method.
+    #[allow(clippy::unused_async)]
+    pub async fn execute(
+        &self,
+        _source: &str,
+        _req: ExecRequest,
+    ) -> Result<ExecResponse, ExecError> {
+        // TODO(executor-core): wire `rhai::Engine::eval_with_scope`
+        Err(ExecError::Runtime(
+            "executor-core::Engine::execute not yet implemented".into(),
+        ))
+    }
+}