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/benevolence.md

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+# Benevolence
+
+## Definition
+
+The disposition to do good to others out of goodwill rather than self-interest.
+Smith argues that benevolence is an insufficient basis for economic organisation
+in a complex society. While a person may secure the friendship of a few through
+appeals to benevolence, they cannot rely on it to obtain the co-operation of
+the "great multitudes" they need in civilised life. Even beggars, who depend
+chiefly on benevolence for their subsistence, conduct most of their actual
+transactions through exchange.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+Benevolence serves as the foil to self-interest. Smith systematically argues
+that while benevolence exists, it cannot scale to support the complex
+interdependencies of a specialised economy, making self-interested exchange
+the necessary coordinating mechanism.
+
+## Economic Domain
+
+General Theory