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/universal-opulence.md

@@ -0,0 +1,34 @@
+# Universal Opulence
+
+## Definition
+
+The general material well-being that extends across all ranks of society,
+including the lowest, as a consequence of the division of labour and the
+resulting multiplication of production. Smith argues that through exchange,
+every workman can supply others abundantly with their specialised product
+and receive in return the products of others' specialisation, creating a
+"general plenty" that benefits even the poorest members of a civilised society.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+The concluding argument of the chapter. Smith illustrates universal opulence
+by examining the "accommodation of the most common artificer or daylabourer,"
+showing that even a coarse woollen coat requires the cooperation of shepherds,
+wool-combers, dyers, weavers, merchants, sailors, and many others — a vast
+chain of interdependent labour that would be impossible without specialisation
+and exchange.
+
+## Economic Domain
+
+Distribution
+
+## Smith's Original Wording
+
+"It is the great multiplication of the productions of all the different arts,
+in consequence of the division of labour, which occasions, in a well-governed
+society, that universal opulence which extends itself to the lowest ranks of
+the people."