docs: rename self-references from "LLM Wiki" to "memex"

Replace project self-references throughout README, SETUP, and the example
CLAUDE.md files. External artifact titles are preserved as-is since they
refer to the actual title of the Claude design artifact.

Also add a "Why 'memex'?" aside to the README that roots the project in
Vannevar Bush's 1945 "As We May Think" essay, where the term originates.
The compounding knowledge wiki is the LLM-era realization of Bush's
memex concept: the "associative trails" he imagined are the related:
frontmatter fields and wikilinks the agent maintains.

Kept lowercase where referring to the generic pattern (e.g. "an LLM wiki
persists its mistakes") since that refers to the class of system, not
this specific project.

docs/SETUP.md

@@ -388,12 +388,12 @@ cat ~/projects/wiki/docs/examples/global-CLAUDE.md >> ~/.claude/CLAUDE.md
 
 This adds:
 - **Wake-Up Context** — read `context/wake-up.md` at session start
-- **LLM Wiki — When to Consult It** — query mode vs ingest mode rules
-- **LLM Wiki — How to Search It** — explicit guidance for all three qmd
+- **memex — When to Consult It** — query mode vs ingest mode rules
+- **memex — How to Search It** — explicit guidance for all three qmd
   collections (`wiki`, `wiki-archive`, `wiki-conversations`) with
   example queries for each
-- **Rules When Citing** — flag `confidence: low`, `status: pending`,
-  and archived pages to the user
+- **memex — Rules When Citing** — flag `confidence: low`,
+  `status: pending`, and archived pages to the user
 
 Together these give the agent a complete picture: how to maintain the
 wiki when working inside it, and how to consult it from anywhere else.