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/power-of-exchanging.md

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+# Power of Exchanging
+
+## Definition
+
+The capacity of economic agents to trade the surplus produce of their own labour for the produce of others. This power is the precondition for the division of labour: without the ability to exchange, there is no incentive to specialise, since a worker cannot consume the entirety of a single specialised output. The power of exchanging is shaped by transportation infrastructure, population density, and the absence of political barriers to trade.
+
+## Source Chapter
+
+Book 1, Chapter 3: "That the Division of Labour is Limited by the Extent of the Market"
+
+## Context
+
+Smith introduces this concept in the chapter's opening sentence as the causal mechanism linking market size to specialisation. It serves as the bridge between the division of labour (Chapter 1-2) and the geographic and infrastructural arguments that follow.
+
+## Economic Domain
+
+Exchange
+
+## Smith's Original Wording
+
+> "As it is the power of exchanging that gives occasion to the division of labour, so the extent of this division must always be limited by the extent of that power."
+
+## Modern Interpretation
+
+This corresponds to the modern concept of market access or trade connectivity — the practical ability of producers to reach buyers, encompassing transaction costs, transportation costs, and institutional barriers to exchange.