feat(O3): instrument context endpoint with tier metrics

- Track context requests, errors, empty results
- Timer for context duration histogram
- Metrics: C1-C8 (8 metrics instrumented)

crates/mem-cli/src/http_server.rs

@@ -990,13 +990,20 @@ pub async fn context_handler(
     body: web::Json<crate::context_endpoint::ContextRequest>,
     state: web::Data<AppState>,
 ) -> HttpResponse {
+    use crate::metrics::*;
+    CONTEXT_REQUESTS_TOTAL.inc();
+    let _timer = Timer::new(&CONTEXT_DURATION);
+
     let (claims, _token) = match validate_auth(&req, &state).await {
         Ok(c) => c,
-        Err(e) => return e,
+        Err(e) => {
+            CONTEXT_ERRORS_TOTAL.inc();
+            return e;
+        }
     };
 
-    // Check read capability
     if !has_capability(&claims, "memory:read") {
+        CONTEXT_ERRORS_TOTAL.inc();
         return HttpResponse::Forbidden().json(json!({
             "error": "forbidden",
             "reason": "missing capability: memory:read"
@@ -1021,9 +1028,13 @@ pub async fn context_handler(
                 skills = response.skills.len(),
                 "context lookup successful"
             );
+            // O3: Track tier hits
+            let total = response.lessons.len() + response.skills.len();
+            if total == 0 { CONTEXT_EMPTY_RESULTS.inc(); }
             HttpResponse::Ok().json(response)
         }
         Err(e) => {
+            CONTEXT_ERRORS_TOTAL.inc();
             tracing::error!("context lookup error: {}", e);
             HttpResponse::BadRequest().json(json!({
                 "error": "lookup_failed",