ai: single-switch GPT/Claude backend for the chat cog

Wire the bot's AI chat pipeline (ai_functions.handle_response) to talk to
either OpenAI or the Anthropic Messages API, chosen by one active-config
switch. Behaviour on the default "gpt" config is unchanged.

constants.py:
* guarded `import anthropic` + CLAUDECLIENT (mirrors OPENAICLIENT), netrc
  machine 'anthropic' / ANTHROPIC_API_KEY;
* CLAUDE_LATEST_MODEL / CLAUDE_CHEAP_MODEL (opus-4-8 / haiku-4-5);
* AI_CONFIGS + DEFAULT_AI_CONFIG loaded from an optional 3rd element of
  system_gpt_settings.json (backward compatible - a 2-element file falls
  back to built-in defaults, active "gpt"). Single switch: CONJURER_AI_CONFIG
  env > settings "active" > "gpt".

ai_functions.py:
* provider_generate() dispatches to OpenAI (unchanged openai_call) or the new
  _anthropic_call() (splits system out, alternating messages, max_tokens,
  temperature omitted - Opus 4.8 rejects sampling params);
* AIError normalises both SDKs' exceptions into one category set so
  handle_response keeps its single set of in-character error replies;
* select_model() reads the active config; legacy "gpt-4o" default auto-maps
  to the active provider's model so the switch actually changes the backend;
* set_active_ai_config()/list_ai_configs() with best-effort persistence back
  into system_gpt_settings.json index 2.

ai_commands.py:
* $gadaj_teraz <config> hybrid command (Vykidailo-gated) switches backend at
  runtime;
* graceful guards when OPENAICLIENT is None: personal assistants (OpenAI
  Assistants API) and DALL-E image gen degrade instead of crashing, so a
  Claude-only deployment boots.

system_gpt_settings.json: add the configs block (gpt/claude/_template) as the
collection point for future backends. requirements_bot.txt: add anthropic.
bot.env.example: ANTHROPIC_API_KEY + CONJURER_AI_CONFIG. Unit tests cover the
message splitter, model selection, config listing, and error mapping.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

system_gpt_settings.json

@@ -39,5 +39,29 @@
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             "asst_1PnD0eQMWo0RkZ3xUacC1uqJ"
         ]
+    },
+    {
+        "active": "gpt",
+        "configs": {
+            "gpt": {
+                "provider": "openai",
+                "latest_model": "gpt-4o",
+                "cheap_model": "gpt-4o-mini",
+                "temperature": 0.2
+            },
+            "claude": {
+                "provider": "anthropic",
+                "latest_model": "claude-opus-4-8",
+                "cheap_model": "claude-haiku-4-5",
+                "max_tokens": 2048
+            },
+            "_template": {
+                "provider": "openai",
+                "latest_model": "model-id-here",
+                "cheap_model": "cheaper-model-id-here",
+                "temperature": 0.2,
+                "max_tokens": 2048
+            }
+        }
     }
 ]