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/improvement-of-art-and-industry.md

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+# Improvement of Art and Industry
+
+## Definition
+
+The progressive advancement of productive techniques, manufacturing methods, and economic organisation that accompanies the expansion of markets. Smith argues that such improvements naturally begin in areas with water-carriage access, where the whole world serves as a potential market, and only later extend to inland regions. The concept links market extent to technological and organisational progress: larger markets incentivise innovation by rewarding specialisation and creating demand for refined products.
+
+## Source Chapter
+
+Book 1, Chapter 3: "That the Division of Labour is Limited by the Extent of the Market"
+
+## Context
+
+This concept appears in the transitional passage between Smith's transport-cost analysis and his historical survey of civilisations. It establishes the causal chain: water-carriage → expanded markets → division of labour → improvement of art and industry. The historical examples (Egypt, Bengal, China) then serve as evidence.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+> "Since such, therefore, are the advantages of water-carriage, it is natural that the first improvements of art and industry should be made where this conveniency opens the whole world for a market to the produce of every sort of labour."
+
+## Modern Interpretation
+
+This concept anticipates endogenous growth theory, which holds that market size affects the rate of innovation. Larger markets increase the returns to developing new techniques, creating a positive feedback loop between market expansion and technological progress.