The most efficient and standard solution is to enable the 'Contacts to Multiple Accounts' feature. This feature allows a single contact record to be associated with multiple account records simultaneously. It creates a related list, 'Related Accounts,' on the contact record and 'Related Contacts' on the account record. This model accurately reflects the real-world scenario where a contractor (Contact) can be associated with different agencies (Accounts) over time. It maintains a single, non-duplicate contact record, preserving the historical accuracy of their affiliations by tracking each relationship individually.

A. Use a partner community to track the Contacts.

A Partner Community is a collaboration platform for external partners, not a data modeling tool for tracking historical employment.

B. Create a new Contact record for each agency.

This creates duplicate contact records for the same individual, leading to data integrity issues and making it impossible to view a unified history.

C. Create a Junction object to track many-to-many relationship.

While technically feasible, this requires a custom solution. The standard 'Contacts to Multiple Accounts' feature provides this functionality declaratively and is the best practice.

---

1. Salesforce Help Documentation: Set Up Contacts to Multiple Accounts.

Reference: This document outlines the steps to enable the feature and states

"Give your sales reps a complete view of who’s involved with their deals by letting them relate a single contact to multiple accounts." This directly addresses the requirement to link one contact to multiple agencies.

2. Salesforce Help Documentation: Considerations for Using Contacts to Multiple Accounts.

Reference: Under the "Relationships" section

it explains how the AccountContactRelation object is used to link a contact with another account. This object allows for defining roles and tracking relationships

which supports the need for "historical accuracy."

3. Trailhead by Salesforce: Accounts and Contacts for Lightning Experience module

Use Contacts to Multiple Accounts unit.

Reference: This unit explicitly states

"Sometimes one person is affiliated with more than one company... Salesforce has a way to handle these complex relationships: Contacts to Multiple Accounts." This confirms it is the designed solution for the scenario described in the question.