feat: add integration tools abstraction and TrustFoundry legal research

From the PR description

Summary

• Add ToolIntegration interface for plugging external tool providers into assistant chat. The current design targets integrations backed by API keys; providers requiring OAuth or per-user token management would need additional key-storage infrastructure.
• Surface integration tool display names in SSE tool_call_start events so the frontend shows user-friendly labels (e.g. "Running TrustFoundry Legal Research...") - display names are defined per-provider, keeping everything encapsulated in a single module
• Return explicit error results for unrecognized tool calls instead of silently dropping them
• Add docs/integrations.md documenting optional third-party integration setup
• Implement TrustFoundry as the first integration: legal search, citation validation, and usage tools

Why

Mike's chat tools were hardcoded - no way to add external tool providers without editing chatTools.ts directly. The ToolIntegration abstraction lets new providers be added as self-contained modules with their own tool definitions, display names, and execution logic.

Test

• npm run build --prefix backend
• npm run build --prefix frontend
• Verified TrustFoundry tools appear in chat when TRUSTFOUNDRY_ENABLED=true and TRUSTFOUNDRY_API_KEY are set; hidden when disabled