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/invention-of-machinery.md

@@ -0,0 +1,31 @@
+# Invention of Machinery
+
+## Definition
+
+The development of machines that facilitate and abridge labour, enabling one
+person to do the work of many. Smith identifies this as the third mechanism
+by which the division of labour increases productive power, and argues that
+the division of labour itself stimulates invention, because workers focused
+on a single operation naturally discover improvements to their specific task.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Presented as the third mechanism. Smith provides the anecdote of the boy who
+automated the valve on a fire engine to free himself for play. He extends the
+argument beyond workers to include machine-makers and philosophers (men of
+speculation), whose own specialised observation enables them to combine
+knowledge from distant fields.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+"Thirdly, and lastly, everybody must be sensible how much labour is facilitated
+and abridged by the application of proper machinery. It is unnecessary to give
+any example."