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/co-operation-of-labour.md

@@ -0,0 +1,31 @@
+# Co-operation of Labour
+
+## Definition
+
+The interdependent collaboration of many workers across different trades and
+locations to produce a single finished good. Smith demonstrates that even the
+simplest consumer goods in a civilised society require the combined efforts of
+thousands of workers — shepherds, miners, sailors, smiths, weavers — who
+collectively make possible what no individual could achieve alone.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Smith's extended example of the day-labourer's woollen coat serves to illustrate
+the vast scope of co-operation. He traces the supply chain from raw materials
+through manufacture and transport to show that civilised consumption depends on
+an immense network of specialised, interdependent labour.
+
+## Economic Domain
+
+Production
+
+## Smith's Original Wording
+
+"Without the assistance and co-operation of many thousands, the very meanest
+person in a civilized country could not be provided, even according to, what we
+very falsely imagine, the easy and simple manner in which he is commonly
+accommodated."