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-philosopher.md

@@ -0,0 +1,31 @@
+# The Philosopher
+
+## Definition
+
+A person whose occupation is observation and speculation rather than direct
+production — "men of speculation, whose trade it is not to do any thing, but
+to observe every thing." Smith treats the philosopher as an economic actor
+whose specialised function is combining knowledge from diverse fields to
+produce innovations and improvements, analogous to how the workman improves
+their own narrow task.
+
+## Source Chapter
+
+Book I, Chapter 1: "Of the Division of Labour"
+
+## Context
+
+Introduced near the end of Smith's discussion of the third mechanism (invention
+of machinery). Smith notes that as society progresses, philosophy itself becomes
+a specialised trade, subdivided into branches, with each philosopher becoming
+expert in their field — the division of labour applied to intellectual work.
+
+## Economic Domain
+
+General Theory
+
+## Smith's Original Wording
+
+"In the progress of society, philosophy or speculation becomes, like every other
+employment, the principal or sole trade and occupation of a particular class of
+citizens."