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/_token_estimator.py

@@ -0,0 +1,16 @@
+"""
+Rough token estimation for backends that don't return usage data.
+
+Uses the ~4 characters per token heuristic common across English LLM tokenizers.
+"""
+
+
+def estimate_tokens(text: str) -> int:
+    """Estimate the number of tokens in *text*.
+
+    This is intentionally coarse — it is only used by the Claude Code CLI
+    adapter where real token counts are unavailable.
+    """
+    if not text:
+        return 0
+    return max(1, len(text) // 4)