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/the-bargain.md

@@ -0,0 +1,32 @@
+# The Bargain
+
+## Definition
+
+A voluntary bilateral exchange in which each party offers something the other
+wants. Smith defines the bargain as the fundamental unit of economic
+interaction: "Give me that which I want, and you shall have this which you
+want." It is through bargaining that individuals obtain "the far greater part
+of those good offices which we stand in need of" in civilised society, as
+opposed to relying on benevolence or coercion.
+
+## Source Chapter
+
+Book I, Chapter 2: "Of the Principle which gives Occasion to the Division
+of Labour"
+
+## Context
+
+The bargain is presented as the practical expression of the propensity to
+exchange. Smith argues that it is the dominant mode of economic interaction,
+used even by beggars who exchange charity-received goods for things they
+actually need.
+
+## Economic Domain
+
+Exchange
+
+## Smith's Original Wording
+
+"Whoever offers to another a bargain of any kind, proposes to do this. Give
+me that which I want, and you shall have this which you want, is the meaning
+of every such offer."