feat(infospace): flat canonical entity set with cross-chapter deduplication

Restructure entity storage from per-chapter subdirectories to a flat
canonical set in output/entities/. Each entity exists as a single file;
duplicates across chapters are detected by slug collision and skipped
(first occurrence wins). Chapter views use {{ include }} transclusion
to reference shared entity files.

Add @{existing_entities} macro to extract-entities template so the LLM
knows which entities already exist and focuses on genuinely new ones.
Refactor _call_llm() from _execute_llm() for callers that handle their
own file I/O. 41 unique entities from 4 chapters (2 duplicates removed).

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

examples/infospace-with-history/output/entities/commercial-society.md

@@ -0,0 +1,13 @@
+# Commercial Society
+
+## Definition
+Commercial Society refers to a society in which the majority of economic activity is based on the exchange of goods and services. In such a society, individuals rely on the production of others for the majority of their needs and wants, facilitated by the use of money as a medium of exchange.
+
+## Source Chapter
+Book 1, Chapter 4
+
+## Context
+Smith uses the concept of a commercial society to explain the development of complex economies where individuals become increasingly specialized in their work and depend on trade with others to meet their needs.
+
+## Economic Domain
+Economic Sociology, Economic History, Microeconomics