feat(mcp): report dynamic server health in context

[wylangyue]

Summary:

• retain a machine-readable last_error_code alongside the existing internal refresh error
• record environment and refresh failures in dynamic MCP server state
• expose status, tool_count, and last_error_code through agentdock_context
• verify idle, ready, and MCP_AUTH_REQUIRED states end to end

This prevents enabled but broken dynamic MCP registrations from being presented as healthy capabilities while keeping raw error messages out of the compact model context.

Dependency:

• blocked on feat(protocol): expose dynamic MCP health fields agentdock-protocol#2, which declares the new strict output fields
• after that PR is merged and released, this PR will update the protocol dependency and be marked ready

Validation against the protocol branch:

• go test ./...
• go vet ./...