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/SD.md

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+# Sequence Diagram (Design) (SD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **OC** — **Required source**: the contract whose postconditions each diagram realizes
+- **DCD** — Forward: classes/methods these messages become
+
+## Required sections (after Metadata / Version History)
+
+One block per realized Operation Contract:
+
+1. **Realizes** — the contract (`[OC-…]`) and its operation name.
+2. **Diagram** — PlantUML sequence diagram: sync (solid filled arrow),
+   async (open arrow), returns (dashed); activations matching the call
+   nesting; `create` / `destroy` shown for transient objects;
+   `loop` / `alt` / `opt` fragments for conditional/repeated behavior.
+3. **Pattern Annotations** — table `Pattern (GRASP/GoF) | Applied to |
+   Rationale`. Patterns are labelled, never implicit.
+4. **Postcondition Coverage** — `Postcondition | Satisfied by message`;
+   every postcondition of the contract must be covered.
+5. **Responsibility Check** — a short note showing no god-object receives
+   all messages (low coupling, high cohesion).
+
+## Terminology
+
+Use the IT terms from the dictionary (`DICT`), not the PO terms the Domain Model uses.