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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.claude/skills/artifact/references/DM.md

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+# Domain Model (DM)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **UC** — Source of every concept (noun phrases) — cite each `UC-*` used
+- **UCD** — Scope check: actors/goals the model must cover
+- **SSD** — Forward: system operations that act on these concepts
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope** — which use cases the model covers.
+2. **Diagram** — PlantUML class diagram (or linked source) showing
+   **concepts, attributes and associations only — no operations**.
+   Business language throughout ("Sale", not "SaleTable"/"SaleClass").
+3. **Concept Table** — `Concept | Definition | Attributes | Source (use
+   case noun phrase / glossary)`. Every concept traces to a noun in a use
+   case or glossary. Attributes are simple domain data, not foreign-key-like
+   references (model those as associations).
+4. **Association Table** — `From | Association name (with reading
+   direction) | To | Multiplicity (both ends)`. All multiplicities present.
+5. **Generalizations** — only true "is-a" relationships, never inheritance
+   for code reuse.
+
+## Terminology
+
+Concept names are the PO terms recorded in the dictionary (`DICT`); add a row there for each new concept.