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/propensity-to-truck-barter-and-exchange.md

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+# Propensity to Truck, Barter, and Exchange
+
+## Definition
+
+An innate or fundamental disposition in human nature to negotiate, trade, and
+exchange goods with others. Smith identifies this propensity as the ultimate
+cause of the division of labour, arguing that it is unique to humans and
+absent in all other animal species. He leaves open whether it is a primary
+instinct or a consequence of the faculties of reason and speech, but treats
+it as the foundational mechanism from which specialisation and economic
+organisation emerge.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+This is the central thesis of the chapter. Smith argues that the division of
+labour "is not originally the effect of any human wisdom" but rather the
+"necessary, though very slow and gradual, consequence" of this propensity.
+The entire chapter serves to establish exchange as the causal origin of
+specialisation.
+
+## Economic Domain
+
+General Theory
+
+## Smith's Original Wording
+
+"This division of labour, from which so many advantages are derived, is not
+originally the effect of any human wisdom, which foresees and intends that
+general opulence to which it gives occasion. It is the necessary, though very
+slow and gradual, consequence of a certain propensity in human nature [...] the
+propensity to truck, barter, and exchange one thing for another."
+
+## Modern Interpretation
+
+This concept prefigures the modern economic assumption of rational self-interest
+as the basis of market behaviour. It also anticipates evolutionary and
+institutional economics debates about whether exchange is a natural disposition
+or a culturally constructed institution.