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/surplus-produce.md

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+# Surplus Produce
+
+## Definition
+
+The portion of a worker's output that exceeds their own consumption needs and
+is therefore available for exchange. Smith argues that the certainty of being
+able to exchange surplus produce for the products of other workers' labour
+is what encourages every person to dedicate themselves to a particular
+occupation. Surplus is thus both the material prerequisite and the incentive
+for specialisation.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+Introduced in the passage describing the emergence of specialised trades in
+a tribal society. The armourer, carpenter, smith, and tanner each produce
+more of their specialty than they can personally consume, and exchange the
+surplus for other goods, reinforcing their commitment to specialisation.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+"And thus the certainty of being able to exchange all that surplus part of
+the produce of his own labour, which is over and above his own consumption,
+for such parts of the produce of other men's labour as he may have occasion
+for, encourages every man to apply himself to a particular occupation."