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/exchange.md

@@ -0,0 +1,32 @@
+# Exchange
+
+## Definition
+
+The act of trading one's surplus production for the goods produced by others.
+Smith presents exchange as the mechanism by which the division of labour
+translates into universal opulence: each workman disposes of their surplus
+output and receives in return the surplus of others, so that all are
+supplied beyond what any individual could produce alone.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Exchange appears in the chapter's conclusion as the connecting mechanism
+between specialised production and general welfare. Smith implicitly treats
+it as prerequisite to the division of labour (explored further in Chapter 2),
+since specialisation only benefits workers if they can trade their surplus.
+
+## Economic Domain
+
+Exchange
+
+## Smith's Original Wording
+
+"Every workman has a great quantity of his own work to dispose of beyond what
+he himself has occasion for; and every other workman being exactly in the same
+situation, he is enabled to exchange a great quantity of his own goods for a
+great quantity or, what comes to the same thing, for the price of a great
+quantity of theirs."