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/the-workman.md

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+# The Workman
+
+## Definition
+
+The individual labourer who performs productive work, whether in manufacturing
+or agriculture. In the context of the division of labour, the workman is the
+operative unit whose dexterity, time, and inventiveness are the channels through
+which specialisation increases output. Smith portrays the workman both as a
+beneficiary of the division of labour (higher output) and as its agent
+(inventing machinery through focused attention).
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+The workman appears throughout the chapter as the primary actor: the pin-maker,
+the nailer, the country weaver, the boy at the fire engine. Smith attributes
+both the productive gains and many mechanical inventions to ordinary workmen.
+
+## Economic Domain
+
+Production