chore: env.example — drop stale Redis stub, document CHARACTERS_DATA_DIR + retention

Redis was dropped as a backend dep in Phase 0 (over a year ago); the
example was carrying placeholder env vars for it. Replace with the
actual two missing env knobs that exist in app/core/config.py but
weren't documented:

- CHARACTERS_DATA_DIR — runtime user-proposed-character mount point
- SCENE_SNAPSHOT_RETENTION — per-scene snapshot prune count

LITELLM_BASE_URL + LITELLM_API_KEY were already documented (Phase 6a).

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

Author: lx-0

.env.example

@@ -36,7 +36,12 @@ SQLITE_URL=sqlite+aiosqlite:////data/sqlite/pixeltales.db  # (default)
 #POSTGRES_DB=postgres
 #POSTGRES_SCHEMA=pixeltales
 
-# Redis settings (for future use)
-#REDIS_HOST=redis
-#REDIS_PORT=6379
-#REDIS_DB=0
+# Character library — runtime data dir for user-proposed characters.
+# Container mounts /data/characters; local dev defaults to data/characters
+# relative to backend/. The seed library at app/agent/characters/ is always
+# scanned in addition to this dir.
+#CHARACTERS_DATA_DIR=/data/characters
+
+# Scene state snapshot retention (per scene_id, default 10).
+# Each new snapshot prunes anything beyond the latest N. 0 disables.
+#SCENE_SNAPSHOT_RETENTION=10