feat(agent): explore MCP tool bridge

[however-yir]

Problem

Agent workflows become more useful when external tools can be connected through a standard integration model instead of project-specific adapters only.

Scope

• Evaluate an MCP bridge for controlled tool access.
• Define how MCP tools interact with tenant permissions, audit logs, and rate limits.
• Document risks around tool safety and secret handling.

Acceptance criteria

• Design notes are added under docs/.
• Security and audit implications are documented.
• A minimal proof-of-concept plan is captured for a later implementation issue.