feat(tools): add update-quest tool, switch to Big Pickle (opencode/zen)

- update_quest: aggiorna stato/nome/descrizione missioni
- db: get_quest(), update_quest con nome+desc
- prompt: quest_id nel contesto, esempio nei tool examples
- Switch da deepseek-v4-flash (go) a big-pickle (zen) — gratuito

src/engine/config.py

@@ -15,9 +15,9 @@ class ProviderConfig:
     openrouter_model: str = "openrouter/free"
     openrouter_key_env: str = "DCC_OPENROUTER_KEY"
     openrouter_free_fallback: bool = True
-    local_base_url: str = "https://opencode.ai/zen/go/v1"
+    local_base_url: str = "https://opencode.ai/zen/v1"
     local_api_key: str = ""
-    local_model: str = "deepseek-v4-flash"
+    local_model: str = "big-pickle"
     timeout: int = 120