p1-2: address spec review (drop user_id_alias mirror in user map)

Spec §"Behavior contract" line 74 says `id:` is captured into
`metadata.user_id_alias` only. Remove the redundant `user.insert`
that was also writing it into the user map, and update the snapshot
baseline accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/kb-parse-md/src/frontmatter.rs

@@ -367,15 +367,9 @@ fn derive_metadata(
     };
 
     // ---- id alias ----
-    // `id:` field becomes `metadata.user_id_alias` AND is mirrored into the
-    // user map under `user_id_alias` (per design §4.2 — not a doc_id factor).
+    // `id:` field becomes `metadata.user_id_alias` only (spec §"Behavior
+    // contract" line 74). It is NOT mirrored into the user map.
     let user_id_alias = raw.id;
-    if let Some(ref id) = user_id_alias {
-        user.insert(
-            "user_id_alias".to_string(),
-            Value::String(id.clone()),
-        );
-    }
 
     Metadata {
         aliases,

fixtures/markdown/frontmatter-only.snapshot.json

@@ -22,8 +22,7 @@
       "original_timestamps": {
         "created_at": "2024-01-15T10:00:00+09:00"
       },
-      "title": "Frontmatter Only",
-      "user_id_alias": "my-stable-handle"
+      "title": "Frontmatter Only"
     }
   },
   "span_present": true,