feat(wiki): concept schema loader + concepts.yaml (F-12)

Adds wiki/concepts.yaml (5 curated seeds: discrete-conformal-map,
circle-packing, lobachevsky-function, discrete-yamabe-flow,
hyperideal-tetrahedron) and codex/wiki.py with full load_concepts()
implementation (Concept dataclass, YAML parser, graceful empty list).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

wiki/concepts.yaml

@@ -0,0 +1,20 @@
+# Schema-Datei: was die KB abdeckt + Emphasis-Hebel (analog CLAUDE.md im Second-Brain).
+# Der Mensch kuratiert diese Liste — sie steuert, worüber die Wiki Seiten baut.
+concepts:
+  - slug: discrete-conformal-map
+    title: Discrete Conformal Map
+    aliases: [discrete conformal equivalence, discrete uniformization]
+    emphasis: Fokus auf die variationelle Charakterisierung (BPS 2015).
+  - slug: circle-packing
+    title: Circle Packing
+    aliases: [Koebe-Andreev-Thurston, circle pattern]
+  - slug: lobachevsky-function
+    title: Lobachevsky Function
+    aliases: [Milnor Lobachevsky, Clausen function]
+    emphasis: Verwendung in hyperbolischen Volumenformeln (Springborn 2008).
+  - slug: discrete-yamabe-flow
+    title: Discrete Yamabe Flow
+    aliases: [discrete Ricci flow, Chow-Luo]
+  - slug: hyperideal-tetrahedron
+    title: Hyperideal Tetrahedron
+    aliases: [hyperbolic volume, Schläfli, Ushijima]