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/KPI.md

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+# KPI Definitions (KPI)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **BC** — The Success Criteria row each KPI is aligned to (`[BC-001]`)
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose** — which Business Case success criteria this document
+   operationalizes.
+2. **KPI Definitions** — one row per KPI: `KPI ID | Name | SMART statement |
+   Baseline | Target | Business Case Success Criterion | Owner | Frequency &
+   Method | Data Source`.
+3. **Thresholds** — per KPI: acceptable / at-risk / failing bounds.
+4. **Reporting** — where results are reported and to whom.
+
+Rules: every KPI is SMART (reject goals/activities like "improve quality");
+baseline *and* target are both present; `Owner` is a stakeholder ID from
+the project's Stakeholder Analysis (e.g. `S07`), never free text; give KPIs stable IDs (`KPI-01`, …) so
+Milestones can cite them.