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/BPMN.md

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+# BPMN Process Model (BPMN)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **BC** — The stated business goal/objective the process serves
+- **SA** — Participants / lanes — cite S-IDs
+- **UCD** — Forward link: actors and goals derived from this process
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Business Goal** — the Business Case objective realized
+   (cite `[BC-001]`).
+2. **Participants (Pools / Lanes)** — every participant, mapped to an `SA`
+   stakeholder ID where one exists.
+3. **Process Diagram** — valid BPMN 2.0. Message flows cross pool
+   boundaries; sequence flows do not. Keep the diagram source next to the
+   document (BPMN has no native markdown form) so it stays diffable.
+4. **Element Table** — `Element | Type | Lane | Description` for every
+   event, activity and gateway. Every gateway states its type (XOR/AND/OR)
+   and its matching join.
+5. **Path Coverage** — every path runs from a start event to a defined end
+   event; no dead ends.