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/SSD.md

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+# System Sequence Diagram (SSD) (SSD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **UC** — **Required source** of every SSD: cite the use case (name and ID) it depicts
+- **DM** — Concepts behind message parameters / returned values
+- **OC** — Forward: contract per system operation shown
+
+## Required sections (after Metadata / Version History)
+
+1. **Source Use Case** — name and ID (`[UC-…]`) and the specific scenario.
+2. **Diagram** — PlantUML sequence diagram with just the actor and
+   `:System`; **no internal objects**. Dashed return arrows for operations
+   that produce a result. One scenario per diagram — separate diagrams for
+   alternate/exception flows (or state them out of scope).
+3. **System Operations Table** — `Step | Message (verb phrase) | Parameters
+   | Return | Use case step`. Messages match the use case's main success
+   scenario step-for-step; justify any deviation. Message names become the
+   Operation Contract names.
+4. **Lifecycle Notes** — creation/destruction of the System instance where
+   relevant (session/transaction scope).