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/US.md

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+# User Story (US)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **UCD** — The story's role must match an actor defined here
+- **UC** — The use case each story traces to
+- **BC** — Objective / epic the story ultimately supports
+- **MIL** — The gateway (epic) each story delivers — one or more stories per gateway
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope** — the epic(s) covered.
+2. **Story List** — each story with a stable ID `US-<doc-version>.<NN>` (e.g.
+   `US-001.01`, so it cannot be confused with the document ID `US-001`):
+   - Statement: **As a** `<actor>`, **I want** `<goal>`, **so that**
+     `<benefit>` — one goal per story, no implementation detail.
+   - **Acceptance Criteria** — clear and testable (Given/When/Then works).
+   - **Traces to** — the use case (`UC-…`) or epic it comes from; a gateway
+     (`MIL-…`) is an epic.
+   - **Size** — fits a single iteration.
+3. **INVEST Check** — one line confirming Independent, Negotiable,
+   Valuable, Estimable, Small, Testable (flag any exception with reason).