* feat: add Z.ai provider support (general API + coding plan)
Add Z.ai (GLM) as a new LLM provider with two variants:
- `zai`: general API (api.z.ai/api/paas/v4)
- `zai_coding`: coding plan (api.z.ai/api/coding/paas/v4)
Reuses OpenAIProvider — Z.ai API is OpenAI-compatible with Bearer
token auth, SSE streaming, and reasoning_content support.
Includes: store constants, config struct fields, env var loading
(GOCLAW_ZAI_API_KEY, GOCLAW_ZAI_CODING_API_KEY), secret masking,
config + DB registration, onboard wizard, and UI provider types.
Default model: glm-5
Closes#100
* docs: add Z.ai provider entries to providers documentation