Add planning baseline: BC, SA, PP and two gateways

Create the Business Case, Stakeholder Analysis (S01 course participant,
S02 Udemy coursists, S03 GitHub viewers), Project Plan and the milestone
documents MIL-001 (project setup, 6 tasks) and MIL-002 (game
implementation, 8 tasks) for the console Blackjack game.

Register PP and MIL in the artifact registry, set the PO language to en,
and link the synced Gitea milestones in the Gateway Schedule.

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.agents/skills/artifact/templates/BC.md

@@ -0,0 +1,70 @@
+# @TITLE@
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | @ID@ |
+| CrossReference | @CROSSREF@ |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| @DATE@ | Accepted | @AUTHOR@ | <reviewer S-ID> | Initial version | pending |
+
+---
+
+## Executive Summary
+
+## Methodological and Standards Foundation
+
+## Problem Statement
+
+## Business Opportunity
+
+## Objectives
+
+## Scope
+
+### In Scope
+
+### Out of Scope
+
+## Expected Benefits
+
+### Tangible Benefits
+
+### Intangible Benefits
+
+## Strategic Alignment
+
+## Success Criteria
+
+| # | Criterion | Target | Measure |
+| --- | --- | --- | --- |
+
+## Risks
+
+| Risk | Impact | Mitigation |
+| --- | --- | --- |
+
+## Assumptions
+
+## Constraints
+
+## Cost–Benefit Assessment
+
+| Costs | Benefits |
+| --- | --- |
+
+## Stakeholders
+
+| Stakeholder ID (SA) | Interest in this project |
+| --- | --- |
+
+## Recommendation
+
+<Proceed | Do not proceed> — <one-sentence rationale>
+
+---
+
+@LINKS@