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/templates/extract-entities.md

@@ -20,22 +20,33 @@ but do not exclude entities simply because they lack an obvious mapping.
 
 @{vsm_framework}
 
+## Existing Entities
+
+The following entities have already been extracted from previous chapters
+of this work. Do NOT re-extract any of these. If one of these entities
+appears in the current chapter, you may omit it entirely — the infospace
+already contains it. Only extract entities that are genuinely new.
+
+@{existing_entities}
+
 ## Instructions
 
 1. Read the source chapter carefully.
-2. Identify all distinct economic concepts, actors, mechanisms, and institutions.
-3. For each entity, produce a separate markdown document following the
+2. Review the list of existing entities above and do not duplicate them.
+3. Identify all distinct economic concepts, actors, mechanisms, and institutions
+   that are NOT already in the existing entities list.
+4. For each new entity, produce a separate markdown document following the
    Economic Entity Schema v1.0.
-4. Each entity document must include:
+5. Each entity document must include:
    - An H1 heading with the entity name
    - A Definition section (20-150 words)
    - A Source Chapter section citing the specific chapter
    - A Context section describing where in the argument the entity appears
    - An Economic Domain section classifying the entity
-5. Optionally include Smith's Original Wording (direct quote) and
+6. Optionally include Smith's Original Wording (direct quote) and
    Modern Interpretation sections.
-6. Use neutral, analytical language throughout.
-7. Ensure each entity is distinct and self-contained.
+7. Use neutral, analytical language throughout.
+8. Ensure each entity is distinct and self-contained.
 
 ## Output Format