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/dexterity-of-the-workman.md

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+# Dexterity of the Workman
+
+## Definition
+
+The skill and speed a worker acquires through repeated performance of a single
+specialised operation. Smith identifies the increase in dexterity as the first
+of three causes by which the division of labour improves productive power.
+Specialisation reduces each worker's task to one simple operation, making it
+the sole employment of their life, and thereby dramatically increasing their
+proficiency.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Presented as the first of three mechanisms explaining why the division of labour
+increases output. Smith illustrates it with the example of nail-making: an
+unskilled smith makes 200-300 nails per day, while a specialised nailer can
+produce over 2,300.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+"First, the improvement of the dexterity of the workmen, necessarily increases
+the quantity of the work he can perform; and the division of labour, by reducing
+every man's business to some one simple operation, and by making this operation
+the sole employment of his life, necessarily increases very much the dexterity
+of the workman."