Write and accept the brief use cases for the P1 candidates

Add the use case diagram UCD-001, the user stories US-001 and the brief use
cases UC-001 to UC-004 (Synchronize Repository Metadata, Configure Workflow
Credentials, Onboard a Repository, Diagnose a Failed Workflow). Review and
accept them (RC-019 to RC-024) and the PP-001 row that records them
(RC-025), and update TM-001 and the registry.

Task: MIL-002#1

docs/uc-002/uc.md

@@ -0,0 +1,43 @@
+# UC-002: Configure Workflow Credentials
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | UC-002 |
+| CrossReference | [UCD-001], [US-001], [SA-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-10 | Accepted | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+**Format:** Brief
+
+**Title:** Configure Workflow Credentials
+**Scope:** Reusable Workflow Collection
+**Level:** user-goal
+**Primary Actor:** DevOps Professional
+**Stakeholders:** S05, S03, S02 (interests below)
+**Stories:** US-001.03, US-001.04
+
+## Brief
+
+The DevOps Professional creates a Gitea token and a GitHub fine-grained token with the minimum permissions in the credentials guide, and the Platform Operator stores them once as secrets of the organization, each holding only the token. A run of the workflow checks that each token is accepted and, for GitHub, that it can administer the mirror, and reports `Preflight passed` without printing any secret value.
+
+**Stakeholder interests**
+
+- S05 — credentials are least-privilege and checked before use
+- S03 — tokens are scoped, stored as secrets and never printed
+- S02 — a failed credential check says which secret to fix
+
+**Preconditions:** The actor has access to the Gitea and GitHub settings and can store secrets for the organization.
+
+**Postconditions (success guarantee):** Both secrets hold only a token, a run passes the credential check with the documented minimum permissions, and no secret value appeared in a log.
+
+---
+
+[UCD-001]: ../use-case-diagram.md
+[US-001]: ../user-stories.md
+[SA-001]: ../stakeholder-analysis.md