Leaderboard wire up live data from agent registry (replaces mock data)

[clintjeff2]

Motivation

• Replace the mock/jittered leaderboard data with real agent data from the registry so leaderboards reflect live agents and earned XP.
• Use XP for the global ranking and task completions for per-district ranking so leaderboard metrics align with the XP and reputation systems.
• Make the leaderboard update promptly for users and show an empty-state when no agents exist.

Description

• Replace the BASE_AGENTS mock with a live call to getAgentRegistry() and enrich each agent with XP via getAgentXP() and task/reputation metrics via getReputation() in lib/leaderboard.ts, and remove the jitter() logic.
• Add getAgentRegistry() export in lib/agent-registry.ts to expose the current in-memory registry.
• Change sorting rules so global view sorts by XP (getAgentXP()), and district view sorts by task completions (from reputation metrics).
• Update API caching for GET /api/leaderboard to return Cache-Control: public, max-age=30 to match the 30s client refresh cadence.
• Adjust client polling in components/leaderboard/leaderboard-table.tsx (remove explicit { cache: "no-store" }) and add an empty-state message shown when no agents are registered.
• Add regression tests in __tests__/api/leaderboard.test.ts covering XP ordering, district filtering/task sorting, empty-state behavior, and cache header.

Testing

• Ran the leaderboard tests with npm test -- --run __tests__/api/leaderboard.test.ts, and all tests passed (3 passed).
• The changes are covered by the new test file __tests__/api/leaderboard.test.ts which validates sorting, district filtering, empty response, and the cache header.

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Closes #195

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[clintjeff2]

@leocagli, review and merge.

[clintjeff2]

@leocagli, please review and merge.