fix: add Kimi Code API support (api.kimi.com/coding/v1)

Kimi Code (platform.kimi.ai) issues API keys prefixed sk-kimi- that require:
1. A different base URL: api.kimi.com/coding/v1 (not api.moonshot.ai/v1)
2. A User-Agent header identifying a recognized coding agent

Without this fix, sk-kimi- keys fail with 401 (wrong endpoint) or 403
('only available for Coding Agents') errors.

Changes:
- Auto-detect sk-kimi- key prefix and route to api.kimi.com/coding/v1
- Send User-Agent: KimiCLI/1.0 header for Kimi Code endpoints
- Legacy Moonshot keys (api.moonshot.ai) continue to work unchanged
- KIMI_BASE_URL env var override still takes priority over auto-detection
- Updated .env.example with correct docs and all endpoint options
- Fixed doctor.py health check for Kimi Code keys

Reference: https://github.com/MoonshotAI/kimi-cli (platforms.py)

run_agent.py

@@ -389,6 +389,12 @@ class AIAgent:
                 "X-OpenRouter-Title": "Hermes Agent",
                 "X-OpenRouter-Categories": "productivity,cli-agent",
             }
+        elif "api.kimi.com" in effective_base.lower():
+            # Kimi Code API requires a recognized coding-agent User-Agent
+            # (see https://github.com/MoonshotAI/kimi-cli)
+            client_kwargs["default_headers"] = {
+                "User-Agent": "KimiCLI/1.0",
+            }
         
         self._client_kwargs = client_kwargs  # stored for rebuilding after interrupt
         try: