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/UCD.md

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+# Use Case Diagram (UCD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **SA** — Each actor traces to a stakeholder need — cite S-IDs
+- **BC** — Scope / objectives the boundary reflects
+- **US** — Forward: stories whose role must match an actor here
+- **UC** — Forward: the use cases detailing each goal shown
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope** — the system boundary in words.
+2. **Diagram** — actors with correct stereotypes (`<<Actor>>`,
+   `<<System>>`), a labelled system boundary, `<<include>>` / `<<extend>>`
+   used per UML 2.5.1 (not as generic "uses"). Embed PlantUML or link the
+   diagram source; no UI or implementation detail.
+3. **Actor Table** — `Actor | Stereotype | Stakeholder ID (SA) | Goals
+   (use cases)`. No orphan actors: every actor appears in at least one use
+   case.
+4. **Use Case Table** — `Use Case | Actor(s) | Goal`, names as **verb
+   phrases describing actor goals** ("Place Order"), not system operations
+   ("Validate Input").
+5. **Relationships** — every `<<include>>` / `<<extend>>` with a one-line
+   justification.