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/components/layout/AppShell.tsx

@@ -144,8 +144,7 @@ const navSections: NavSection[] = [
   {
     label: "Analytics",
     items: [
-      { href: "/analytics/skills", label: "Skills Analytics", icon: <SkillsIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER", "VIEWER"] },
-      { href: "/analytics/skill-marketplace", label: "Skill Marketplace", icon: <MarketplaceIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
+      { href: "/analytics/skills", label: "Skills Hub", icon: <SkillsIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER", "VIEWER"] },
       { href: "/reports/chargeability", label: "Chargeability", icon: <ChargeabilityIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
       { href: "/reports/builder", label: "Report Builder", icon: <ReportBuilderIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },
       { href: "/analytics/computation-graph", label: "Computation Graph", icon: <GraphIcon />, roles: ["ADMIN", "MANAGER", "CONTROLLER"] },