fix: improve Kimi model selection — auto-detect endpoint, add missing models

Kimi Coding Plan setup:
- New dedicated _model_flow_kimi() replaces the generic API-key flow
  for kimi-coding. Removes the confusing 'Base URL' prompt entirely —
  the endpoint is auto-detected from the API key prefix:
    sk-kimi-* → api.kimi.com/coding/v1 (Kimi Coding Plan)
    other     → api.moonshot.ai/v1 (legacy Moonshot)

- Shows appropriate models for each endpoint:
    Coding Plan: kimi-for-coding, kimi-k2.5, kimi-k2-thinking, kimi-k2-thinking-turbo
    Moonshot:    full model catalog

- Clears any stale KIMI_BASE_URL override so runtime auto-detection
  via _resolve_kimi_base_url() works correctly.

Model catalog updates:
- Added kimi-for-coding (primary Coding Plan model) and kimi-k2-thinking-turbo
  to models.py, main.py _PROVIDER_MODELS, and model_metadata.py context windows.

- Updated User-Agent from KimiCLI/1.0 to KimiCLI/1.3 (Kimi's coding
  endpoint whitelists known coding agents via User-Agent sniffing).

agent/model_metadata.py

@@ -53,8 +53,10 @@ DEFAULT_CONTEXT_LENGTHS = {
     "glm-5": 202752,
     "glm-4.5": 131072,
     "glm-4.5-flash": 131072,
+    "kimi-for-coding": 262144,
     "kimi-k2.5": 262144,
     "kimi-k2-thinking": 262144,
+    "kimi-k2-thinking-turbo": 262144,
     "kimi-k2-turbo-preview": 262144,
     "kimi-k2-0905-preview": 131072,
     "MiniMax-M2.5": 204800,