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/manufactures.md

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+# Manufactures
+
+## Definition
+
+The sector of production in which raw materials are transformed into finished
+goods through a series of distinct operations, each typically performed by
+specialised workers. Smith contrasts manufactures with agriculture, noting that
+the former admits of far greater subdivision of labour and separation of trades,
+and therefore exhibits far greater improvements in productive power.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Manufactures serve as the primary setting for Smith's analysis of the division
+of labour. The pin factory is a manufacture; so are the linen, woollen, and
+hardware trades he references. Smith uses the greater divisibility of
+manufacturing work to explain why rich countries excel more conspicuously over
+poor countries in manufactures than in agriculture.
+
+## Economic Domain
+
+Production