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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.claude/skills/artifact/references/SA.md

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+# Stakeholder Analysis (SA)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **BC** — Business Case objectives each stakeholder concern traces to (Business Goal Alignment section)
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose** — why the analysis exists and the methodology it follows.
+2. **Stakeholder Summary Table** — `ID | Name | Role/Title | Organization |
+   Power Level | Interest Level | Quadrant | Primary Concern (Business
+   Language)`. Every row fully filled; no unclassified stakeholder.
+3. **Power/Interest Classification Rationale** — narrative per quadrant,
+   consistent with the table.
+4. **Primary Concerns and FURPS+ Mapping** — each concern in business
+   language *and* mapped to a FURPS+ attribute.
+5. **Communication Requirements** — channel, frequency, deliverable type,
+   tied to a project phase or milestone.
+6. **Conflicting Interests and Mitigations** — every conflict has a
+   mitigation.
+7. **Traceability Analysis** — stakeholder → actor/use case mapping, and
+   business-goal alignment citing Business Case (`[BC-001]`) objectives.
+8. **Sign-Off**.
+
+Stakeholder IDs (`S01`, `S02`, …) are **stable: never renumbered or reused**.
+Every other artifact cites them for RACI, ownership and review assignment
+(`AGENTS.md` rule 3). Add new stakeholders with the next free `S<NN>`.