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>

docker/env/bot.env.example

@@ -1,15 +1,22 @@
 # Copy to docker/env/bot.env and fill in. Do NOT commit the real file.
 
 # --- Secrets ------------------------------------------------------------
-# Option A: mount a netrc (recommended — covers discord/openai/spotipy/youtube).
+# Option A: mount a netrc (recommended — covers discord/openai/anthropic/spotipy/youtube).
 CONJURER_NETRC_FILE=/secrets/.netrc
 # Option B: pass tokens directly (these take precedence over netrc).
 # DISCORD_TOKEN=
 # OPENAI_API_KEY=
+# ANTHROPIC_API_KEY=       # Claude backend; netrc machine 'anthropic' works too
 # ASSEMBLYAI_API_KEY=      # voice recognition; netrc machine 'assemblyai' works too
 # YOUTUBE_USERNAME=
 # YOUTUBE_PASSWORD=
 
+# --- AI backend switch --------------------------------------------------
+# Which AI config from system_gpt_settings.json is active at startup
+# (e.g. "gpt" or "claude"). Runtime switch: $gadaj_teraz <config>. Unset =
+# whatever the settings file's "active" key says, falling back to "gpt".
+# CONJURER_AI_CONFIG=gpt
+
 # --- Data ---------------------------------------------------------------
 # Single mounted volume; all writable state is rooted here.
 CONJURER_DATA_DIR=/data