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/division-of-labour.md

@@ -0,0 +1,39 @@
+# Division of Labour
+
+## Definition
+
+The separation of a work process into a number of distinct tasks, each performed
+by a specialised worker, resulting in a significant increase in the productive
+powers of labour. Smith identifies it as the principal cause of improvement in
+the productive capacity of any trade, art, or manufacture. The effect arises
+from three circumstances: increased dexterity, saved time in transition between
+tasks, and the invention of labour-saving machinery.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+The division of labour is the central argument of the chapter. Smith opens by
+asserting that it is the greatest source of improvement in productive powers,
+then illustrates it through the pin-factory example, explains its three causal
+mechanisms, and concludes by showing how it generates universal opulence through
+exchange.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+"The greatest improvements in the productive powers of labour, and the greater
+part of the skill, dexterity, and judgment, with which it is anywhere directed,
+or applied, seem to have been the effects of the division of labour."
+
+## Modern Interpretation
+
+The division of labour remains a foundational concept in economics and
+organisational theory. Modern extensions include specialisation theory,
+comparative advantage (Ricardo), and the study of transaction costs that
+determine the boundaries between internal division and market exchange (Coase).