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/common-stock.md

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+# Common Stock
+
+## Definition
+
+The aggregate pool of goods and services created when individuals bring
+their diverse specialised products together through exchange. Smith argues
+that among humans, unlike animals, different talents are made useful to
+one another because their products can be "brought, as it were, into a
+common stock, where every man may purchase whatever part of the produce
+of other men's talents he has occasion for." This common stock is the
+emergent result of widespread exchange among specialised producers.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+Appears in the chapter's concluding argument comparing humans and animals.
+While a mastiff cannot benefit from a greyhound's speed due to lack of
+exchange, humans can pool their different abilities through trade, making
+all talents contribute to the general welfare.
+
+## Economic Domain
+
+Exchange