feat(api): migrate from llama-cli subprocess to llama-server REST API

feat(api): migrate from llama-cli subprocess to llama-server REST API

Replace local llama-cli binary execution with HTTP calls to a running
llama-server instance. This enables KV cache reuse across turns, which is
essential for future multi-turn agent mode.

Key changes:
- TaskProcess now POSTs to /v1/chat/completions instead of spawning process
- Skill format changed from raw prompt template to system_prompt field
- Configuration simplified: removed llamaCliPath, thread counts, modelsDirectory,
  and all sampling defaults; added server_url
- Wizard stripped of model discovery and llama-cli path validation
- Added --once flag to process command for one-shot batch execution
- Removed all getEffective* sampling parameter methods from Task