Add gated Gravity Index tool

[brandonkachen]

Summary

• Add native gravity_index tool, but keep it gated out of base/free agent tool lists for initial rollout
• Consolidate Gravity Index actions behind one tool: search, browse, list_categories, get_service, and report_integration
• Proxy Gravity Index through /api/v1/gravity-index, using server-side GRAVITY_API_KEY only for publisher-key actions
• Fold guidance from @gravity-ai/index-mcp tool prompts into the native tool prompt, including setup-link handling
• Share the Gravity input schema between the native tool and API route, and fix generated tool typings for root union schemas
• Add runtime, API, SDK E2E, and tool-definition generator coverage; SDK E2E opts into the tool with a test-only agent

Tests

• bun test agents/e2e/gravity-index.e2e.test.ts common/src/tools/__tests__/compile-tool-definitions.test.ts web/src/app/api/v1/gravity-index/__tests__/gravity-index.test.ts packages/agent-runtime/src/__tests__/gravity-index-tool.test.ts
• bun run --cwd agents typecheck
• bun run --cwd common typecheck
• bun run --cwd web typecheck
• bun run --cwd packages/agent-runtime typecheck