feat(llm): add LLM integration module with OpenRouter and Claude Code adapters

Implements markitect/llm/ package with concrete LLMAdapter implementations:
- OpenRouterAdapter: HTTP via urllib with retry/backoff on 429/5xx
- ClaudeCodeAdapter: subprocess-based Claude CLI with stdin piping
- Factory pattern: create_adapter("openrouter") or create_adapter("claude-code")
- API key resolution chain: constructor > env var > project-root key file
- 42 unit tests, 2 integration tests (gated on API key / CLI availability)

Also adds the infospace-with-history example with Wealth of Nations VSM
analysis pipeline, templates, schemas, source chapters, and processed
output for chapters 1-2. process_chapters.py now supports --provider
and --model flags for automatic LLM-driven processing.

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

markitect/llm/exceptions.py

@@ -0,0 +1,71 @@
+"""
+LLM-specific exceptions.
+
+Extends the MarkitectError hierarchy for LLM integration errors.
+"""
+
+from typing import Optional, Dict, Any
+
+from markitect.exceptions import MarkitectError
+
+
+class LLMError(MarkitectError):
+    """Base exception for all LLM operations."""
+    pass
+
+
+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