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/_http.py

@@ -0,0 +1,79 @@
+"""
+Thin synchronous HTTP helper built on :mod:`urllib.request`.
+
+Translates HTTP errors into typed :mod:`markitect.llm.exceptions`.
+"""
+
+import json
+import urllib.request
+import urllib.error
+from typing import Dict, Any, Optional
+
+from markitect.llm.exceptions import (
+    LLMAPIError,
+    LLMRateLimitError,
+    LLMTimeoutError,
+)
+
+
+def post_json(
+    url: str,
+    payload: Dict[str, Any],
+    headers: Optional[Dict[str, str]] = None,
+    timeout: int = 300,
+) -> Dict[str, Any]:
+    """POST *payload* as JSON and return the parsed response body.
+
+    Raises:
+        LLMRateLimitError: on HTTP 429
+        LLMAPIError: on other non-2xx responses
+        LLMTimeoutError: on socket / read timeout
+    """
+    data = json.dumps(payload).encode()
+    req = urllib.request.Request(
+        url,
+        data=data,
+        headers={"Content-Type": "application/json", **(headers or {})},
+        method="POST",
+    )
+
+    try:
+        with urllib.request.urlopen(req, timeout=timeout) as resp:
+            body = resp.read().decode()
+            return json.loads(body)
+    except urllib.error.HTTPError as exc:
+        body = ""
+        try:
+            body = exc.read().decode()
+        except Exception:
+            pass
+
+        if exc.code == 429:
+            raise LLMRateLimitError(
+                f"Rate limited (429) from {url}",
+                status_code=429,
+                response_body=body,
+                cause=exc,
+            ) from exc
+
+        raise LLMAPIError(
+            f"HTTP {exc.code} from {url}",
+            status_code=exc.code,
+            response_body=body,
+            cause=exc,
+        ) from exc
+    except urllib.error.URLError as exc:
+        if "timed out" in str(exc.reason):
+            raise LLMTimeoutError(
+                f"Request to {url} timed out after {timeout}s",
+                cause=exc,
+            ) from exc
+        raise LLMAPIError(
+            f"URL error for {url}: {exc.reason}",
+            cause=exc,
+        ) from exc
+    except TimeoutError as exc:
+        raise LLMTimeoutError(
+            f"Request to {url} timed out after {timeout}s",
+            cause=exc,
+        ) from exc