refactor: centralize section config — remove all hardcoded section lists

- New packages/config/src/sections.ts: SECTIONS, SECTIONS_BY_ID, SECTION_IDS
  (no Node.js built-in imports — safe for client components)
- New package entrypoint: @mcpedia/config/sections (exports only sections.ts)
- Client components import from @mcpedia/config/sections (no node:fs in bundle)
- Server components import from @mcpedia/config (has env/fs, server-only)
- Removed hardcoded SECTIONS/SECTION_LABEL/SECTION_ICON from:
  - [section]/page.tsx (was inline array)
  - [section]/[...slug]/page.tsx (was hardcoded Record maps)
  - Sidebar.tsx (was inline array)
  - layout.tsx (was inline array)
  - page.tsx (was inline array)
  - create/page.tsx (was inline array via SECTIONS import)
- Added @mcpedia/config/sections entrypoint to package.json exports

packages/config/src/sections.ts

@@ -0,0 +1,44 @@
+/**
+ * Section registry — defines all content sections for the UI.
+ * This file is intentionally free of Node.js built-in imports (no fs/path)
+ * so it can be safely imported by client components.
+ *
+ * Adding a new section is as simple as adding an entry here + creating a
+ * `content/<id>/` directory with markdown files. No UI code changes needed.
+ */
+export interface SectionConfig {
+  id: string;
+  label: string;
+  icon: string;
+  desc: string;
+}
+
+export const SECTIONS: SectionConfig[] = [
+  {
+    id: "docs",
+    label: "Documentation",
+    icon: "📄",
+    desc: "Setup guides, API references, and protocol specs.",
+  },
+  {
+    id: "writeups",
+    label: "Writeups",
+    icon: "📝",
+    desc: "Post-mortems, debugging stories, and case studies.",
+  },
+  {
+    id: "research",
+    label: "Research",
+    icon: "🔬",
+    desc: "Deep-dive analysis, architecture notes, and experiments.",
+  },
+  {
+    id: "notes",
+    label: "Notes",
+    icon: "📌",
+    desc: "Quick references, patterns, and gotchas.",
+  },
+];
+
+export const SECTIONS_BY_ID = new Map(SECTIONS.map((s) => [s.id, s]));
+export const SECTION_IDS = SECTIONS.map((s) => s.id);