feat: initial llm-connect package scaffold

Copy markitect.llm module into standalone llm_connect package.
All markitect.* imports replaced with llm_connect.* equivalents.
LLMError base class inlined (no markitect.exceptions dependency).
Verified: from llm_connect import create_adapter works.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

llm_connect/exceptions.py

@@ -0,0 +1,85 @@
+"""
+LLM-specific exceptions.
+"""
+
+from typing import Optional, Dict, Any
+
+
+class LLMError(Exception):
+    """Base exception for all LLM operations."""
+
+    def __init__(
+        self,
+        message: str,
+        cause: Optional[Exception] = None,
+        context: Optional[Dict[str, Any]] = None,
+    ):
+        super().__init__(message)
+        self.cause = cause
+        self.context = context or {}
+        if cause:
+            self.__cause__ = cause
+
+    def __str__(self) -> str:
+        base = super().__str__()
+        if self.context:
+            ctx = ", ".join(f"{k}={v}" for k, v in self.context.items())
+            base = f"{base} [Context: {ctx}]"
+        return base
+
+
+class LLMConfigurationError(LLMError):
+    """Missing API key, invalid model name, or bad provider config."""
+    pass
+
+
+class LLMAPIError(LLMError):
+    """HTTP-level failure from an LLM provider API.
+
+    Attributes:
+        status_code: HTTP status code (e.g. 500, 502).
+        response_body: Raw response body text, if available.
+    """
+
+    def __init__(
+        self,
+        message: str,
+        status_code: int = 0,
+        response_body: str = "",
+        cause: Optional[Exception] = None,
+        context: Optional[Dict[str, Any]] = None,
+    ):
+        super().__init__(message, cause=cause, context=context)
+        self.status_code = status_code
+        self.response_body = response_body
+
+
+class LLMRateLimitError(LLMAPIError):
+    """429 Too Many Requests from the provider."""
+    pass
+
+
+class LLMTimeoutError(LLMError):
+    """Request or subprocess exceeded the configured timeout."""
+    pass
+
+
+class LLMSubprocessError(LLMError):
+    """Claude Code CLI subprocess failed.
+
+    Attributes:
+        return_code: Process exit code.
+        stderr: Captured stderr text.
+    """
+
+    def __init__(
+        self,
+        message: str,
+        return_code: int = 1,
+        stderr: str = "",
+        cause: Optional[Exception] = None,
+        context: Optional[Dict[str, Any]] = None,
+    ):
+        super().__init__(message, cause=cause, context=context)
+        self.return_code = return_code
+        self.stderr = stderr