feat(web): expose Kepos Bridge in MCP and Pi

[orga-agent]

Summary

• Expose Kepos Bridge provider selection in CLI and stdio MCP.
• Allow Pi to pin its search provider via dependency configuration or WEB_SEARCH_PROVIDER.
• Keep MCP and Pi on the core default Bridge route; DSH remains the only user-facing route configuration.

Verification

• pnpm run typecheck
• pnpm test
• pnpm run format:check