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/references/DCD.md

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+# Design Class Diagram (DCD) (DCD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **DM** — Concepts each design class refines — names must stay consistent
+- **SD** — Messages that become method signatures
+- **ERD** — Forward: persistence of the classes' attributes
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope**.
+2. **Diagram** — PlantUML class diagram: visibility markers (`+` `-` `#`)
+   on every member; association vs aggregation vs composition vs dependency
+   used per true ownership/lifecycle; multiplicity and navigability on every
+   association.
+3. **Class Table** — `Class | Refines (Domain Model concept) |
+   Responsibility (one sentence) | Attributes | Operations`. SOLID applied;
+   no god classes; names consistent with the Domain Model.
+4. **Method Traceability** — `Method signature | Operation Contract / SD
+   message`; every method traces to one.
+5. **Pattern Annotations** — `Pattern | Classes | Rationale`, explicit.
+6. **Dependency Check** — note confirming no circular class/package
+   dependencies (or an explicit justification).
+
+## Terminology
+
+Class and attribute names are the IT terms from the dictionary (`DICT`), not the PO terms the Domain Model uses.