feat: add MiniMax TTS provider support (speech-2.8)

Add MiniMax as a fifth TTS provider alongside Edge TTS, ElevenLabs,
OpenAI, and NeuTTS. Supports speech-2.8-hd (recommended default) and
speech-2.8-turbo models via the MiniMax T2A HTTP API.

Changes:
- Add _generate_minimax_tts() with hex-encoded audio decoding
- Add MiniMax to provider dispatch, requirements check, and Telegram
  Opus compatibility handling
- Add MiniMax to interactive setup wizard with API key prompt
- Update TTS documentation and config example

Configuration:
  tts:
    provider: "minimax"
    minimax:
      model: "speech-2.8-hd"
      voice_id: "English_Graceful_Lady"

Requires MINIMAX_API_KEY environment variable.

API reference: https://platform.minimax.io/docs/api-reference/speech-t2a-http

cli-config.yaml.example

@@ -539,7 +539,7 @@ platform_toolsets:
 #   skills_hub   - skill_hub (search/install/manage from online registries — user-driven only)
 #   moa          - mixture_of_agents  (requires OPENROUTER_API_KEY)
 #   todo         - todo (in-memory task planning, no deps)
-#   tts          - text_to_speech  (Edge TTS free, or ELEVENLABS/OPENAI key)
+#   tts          - text_to_speech  (Edge TTS free, or ELEVENLABS/OPENAI/MINIMAX key)
 #   cronjob      - cronjob (create/list/update/pause/resume/run/remove scheduled tasks)
 #   rl           - rl_list_environments, rl_start_training, etc. (requires TINKER_API_KEY)
 #
@@ -568,7 +568,7 @@ platform_toolsets:
 #   todo         - Task planning and tracking for multi-step work
 #   memory       - Persistent memory across sessions (personal notes + user profile)
 #   session_search - Search and recall past conversations (FTS5 + Gemini Flash summarization)
-#   tts          - Text-to-speech (Edge TTS free, ElevenLabs, OpenAI)
+#   tts          - Text-to-speech (Edge TTS free, ElevenLabs, OpenAI, MiniMax)
 #   cronjob      - Schedule and manage automated tasks (CLI-only)
 #   rl           - RL training tools (Tinker-Atropos)
 #