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/difference-of-talents.md

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+# Difference of Talents
+
+## Definition
+
+The observable variation in skills, aptitudes, and abilities among individuals
+in different occupations. Smith makes the striking argument that this
+difference is largely the effect rather than the cause of the division of
+labour: people are born with roughly equal abilities, and it is their
+different occupations, shaped by habit, custom, and education, that create
+the apparent differences. He contrasts humans with dogs, where natural breed
+differences are far greater but cannot be made useful because animals lack
+the capacity for exchange.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+This argument occupies the final portion of the chapter. Smith uses it to
+reinforce his claim that exchange, not innate difference, is the driver of
+specialisation. The philosopher and the street porter were "very much alike"
+until different employments shaped them differently.
+
+## Economic Domain
+
+General Theory
+
+## Smith's Original Wording
+
+"The difference of natural talents in different men, is, in reality, much
+less than we are aware of; and the very different genius which appears to
+distinguish men of different professions, when grown up to maturity, is not
+upon many occasions so much the cause, as the effect of the division of
+labour."