feat: unified Skills Hub — merge analytics + marketplace into one page

Combines SkillsAnalytics (496 LOC) and SkillMarketplace (346 LOC) into
a single tabbed Skills Hub (770 LOC total, -9% code).

New structure:
- skills/shared.tsx: ProficiencyBadge, GapIndicator, constants (extracted)
- skills/OverviewTab.tsx: KPI cards, top 10 table, distribution chart, export
- skills/SearchTab.tsx: skill search + proficiency + availability filter
- skills/GapsTab.tsx: supply vs demand table with gap indicators
- skills/PeopleFinderTab.tsx: multi-rule AND/OR builder, chapter filter, export
- SkillsHub.tsx: tabbed container with URL-persisted tab state (?tab=)

Routing:
- /analytics/skills renders SkillsHub (was SkillsAnalytics)
- /analytics/skill-marketplace redirects to /analytics/skills?tab=search
- Sidebar: "Skill Marketplace" removed, renamed to "Skills Hub"

No API changes — reuses existing queries with conditional fetching per tab.
Full dark theme support on all components.

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/app/(app)/analytics/skill-marketplace/page.tsx

@@ -1,5 +1,5 @@
-import { SkillMarketplace } from "~/components/analytics/SkillMarketplace.js";
+import { redirect } from "next/navigation";
 
 export default function SkillMarketplacePage() {
-  return <SkillMarketplace />;
+  redirect("/analytics/skills?tab=search");
 }

apps/web/src/app/(app)/analytics/skills/page.tsx

@@ -1,5 +1,5 @@
-import { SkillsAnalytics } from "~/components/analytics/SkillsAnalytics.js";
+import { SkillsHub } from "~/components/analytics/SkillsHub.js";
 
-export default function SkillsAnalyticsPage() {
-  return <SkillsAnalytics />;
+export default function SkillsHubPage() {
+  return <SkillsHub />;
 }