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/TM.md

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+# Traceability Matrix (TM)
+
+One matrix per project: `docs/sqa/traceability-matrix.md`. It makes the
+Business Case's cross-artifact traceability target measurable.
+
+## Required sections (after Metadata / Version History)
+
+- **Purpose**.
+- **Traceability Table** — one row per artifact instance:
+  `Artifact Instance | Type | Upstream (Backward Link) | Downstream (Forward
+  Link) | Last Reviewed (RC-ID)`. Add or update a row whenever an instance
+  is created or reviewed.
+- **Coverage Notes** — which types have no instance yet, and how to read
+  `-`: in Upstream it means foundational; in Downstream it means nothing
+  is built on it yet; in Last Reviewed it means no `RC-*` exists yet.