Feat v0.6.15 user display audit

Add batch user resolve endpoint, sw.users SDK module, and migrate all
surfaces to show display_name instead of UUIDs. Canonical fallback chain:
display_name → username → "Unknown".

- GET /api/v1/users/resolve?ids=... (max 100, map response)
- sw.users.resolve(), resolveMany(), displayName() with 60s cache
- Chat participants resolved from users table, not snapshot
- Admin users/teams/groups/team-admin show display_name
- Chat-core participants.display_name column deprecated
- 5 new handler tests, OpenAPI spec updated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/chat-core/script.star

@@ -91,6 +91,9 @@ def create(title, type="group", participants=None, creator_id="", creator_displa
     cid = conv["id"]
 
     # Add creator as admin participant
+    # NOTE: display_name is a snapshot captured at creation time.
+    # DEPRECATED v0.6.15 — UI resolves display names from users table via sw.users.resolve().
+    # Column retained for backward compatibility; UI no longer relies on this value for display.
     if creator_id:
         db.insert("participants", {
             "conversation_id": cid,
@@ -110,7 +113,7 @@ def create(title, type="group", participants=None, creator_id="", creator_displa
             "conversation_id": cid,
             "participant_id": pid,
             "participant_type": _str(p.get("type", "user")),
-            "display_name": _str(p.get("display_name", "")),
+            "display_name": _str(p.get("display_name", "")),  # DEPRECATED v0.6.15 — snapshot only
             "role": _str(p.get("role", "member")),
             "joined_at": "",
         })
@@ -213,7 +216,7 @@ def add_participant(conversation_id, participant_id, participant_type="user", di
         "conversation_id": cid,
         "participant_id": pid,
         "participant_type": _str(participant_type),
-        "display_name": _str(display_name),
+        "display_name": _str(display_name),  # DEPRECATED v0.6.15 — snapshot only
         "role": _str(role),
         "joined_at": "",
     })