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/TRN.md

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+# Reviewer Training (TRN)
+
+The reusable training material for reviewers. It mitigates the "resistance to
+standardized reviews" risk of the Business Case. Framework-level, like `PRC`:
+no project data, no stakeholder IDs, no links to project artifacts. The
+*record* that a session happened (date, attendees, result) is a project
+document, written after the session.
+
+- **File:** `framework/process/reviewer-training.md` (singleton, version `001`).
+- **Create:** `new-artifact.sh TRN`.
+- **CrossReference:** `PRC` (the procedure the training teaches).
+
+## Required sections (after Metadata / Version History)
+
+Purpose, Audience and Prerequisites, Learning Objectives, Agenda (modules with
+minutes), Module Notes, Exercises (each with input and expected result),
+Assessment (how and pass criteria), Session Record (what to capture).
+Exercises use a seeded, generic example, not a real project artifact.