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/ERD.md

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+# Entity Relationship Diagram (ERD) (ERD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **DCD** — **Required source**: classes/attributes each entity persists; data types must match
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope**.
+2. **Diagram** — PlantUML entity diagram with PK/FK marked on every entity and
+   cardinality (1:1, 1:N) on every relationship; N:M relationships resolved
+   through explicit junction entities.
+3. **Entity Table** — per entity: `Attribute | Type | PK/FK | Nullable |
+   Source (DCD class.attribute)`. Types consistent with the DCD; consistent
+   naming, no implementation-specific abbreviations.
+4. **Relationship Table** — `Entity | Cardinality | Entity | FK | Rule`.
+5. **Normalization Notes** — 3NF confirmed; any denormalization documented
+   with its performance justification.
+
+## Terminology
+
+Entity and column names follow the IT terms from the dictionary (`DICT`), not the PO terms the Domain Model uses.