worker: rename internal worker purpose to slug

.yoi/tickets/00001KXRXAJFZ/item.md

@@ -27,7 +27,7 @@ Protocol に外部 API として載せる段階ではない。まず Worker 内
 
 - Worker 内部に reusable internal worker runner を追加する。
 - runner は少なくとも次を受け取れる。
-  - purpose / name。
+  - slug / name。
   - system prompt。
   - initial user input。
   - tool definitions / limited tool registry。

crates/worker/src/internal_worker.rs

@@ -14,8 +14,8 @@ use llm_engine::{Engine, EngineError};
 
 /// Specification for a single internal worker run.
 pub(crate) struct InternalWorkerSpec {
-    /// Stable purpose label for audit/debug/persistence metadata.
-    pub purpose: &'static str,
+    /// Stable slug for audit/debug/future persistence metadata.
+    pub slug: &'static str,
     /// System prompt for the isolated engine.
     pub system_prompt: String,
     /// Initial user input for the isolated engine.
@@ -33,7 +33,7 @@ pub(crate) struct InternalWorkerSpec {
 /// Result metadata for an internal worker run.
 #[derive(Debug, Clone, Default)]
 pub(crate) struct InternalWorkerRunResult {
-    pub purpose: &'static str,
+    pub slug: &'static str,
     /// Last usage event observed for this internal run.
     pub usage: Option<UsageEvent>,
 }
@@ -41,7 +41,7 @@ pub(crate) struct InternalWorkerRunResult {
 /// Error metadata for an internal worker run.
 #[derive(Debug)]
 pub(crate) struct InternalWorkerRunError {
-    pub purpose: &'static str,
+    pub slug: &'static str,
     pub source: EngineError,
     /// Last usage event observed before the failure, if any.
     pub usage: Option<UsageEvent>,
@@ -52,7 +52,7 @@ pub(crate) async fn run_internal_worker(
     spec: InternalWorkerSpec,
 ) -> Result<InternalWorkerRunResult, InternalWorkerRunError> {
     let InternalWorkerSpec {
-        purpose,
+        slug,
         system_prompt,
         input,
         client,
@@ -82,9 +82,9 @@ pub(crate) async fn run_internal_worker(
         .clone();
 
     match run_result {
-        Ok(_output) => Ok(InternalWorkerRunResult { purpose, usage }),
+        Ok(_output) => Ok(InternalWorkerRunResult { slug, usage }),
         Err(source) => Err(InternalWorkerRunError {
-            purpose,
+            slug,
             source,
             usage,
         }),

crates/worker/src/worker.rs

@@ -3133,7 +3133,7 @@ impl<C: LlmClient, St: Store> Worker<C, St> {
         let ctx = Arc::new(extract::ExtractWorkerContext::new());
         let input_text = extract::build_extract_input(&items_to_extract);
         let internal_result = run_internal_worker(InternalWorkerSpec {
-            purpose: "memory_extract",
+            slug: "memory-extract",
             system_prompt: extract_system_prompt,
             input: input_text,
             client,