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/ADR.md

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+# Architecture Decision Record (ADR)
+
+Files: `docs/adr/adr-NNNN-kebab-case-title.md`. `NNNN` is 4-digit and
+sequential; the ID is `ADR-NNNN` and must match the filename number (an
+intentional exception to the usual 3-digit IDs). Use
+`new-artifact.sh ADR --file docs/adr/adr-NNNN-title.md`.
+
+Cite in `CrossReference` the artifacts this decision is about (pass each
+with `--cite <ID>=<path>`); ADR has no fixed candidate list.
+
+## Required sections (after Metadata / Version History)
+
+- **Context** — the problem, the options evaluated and the forces
+  (cost, risk, constraints).
+- **Decision** — the outcome in one or two sentences, no hedging.
+- **Consequences** — two bold labels, **Positive:** and **Negative:**, each
+  with a bullet list.
+- **Affected Artifacts** — other artifacts impacted, as `[ID]` links, or a
+  single `-` if none.
+
+Allowed statuses: `Proposed`, `Accepted`, `Rejected`, `Deprecated`,
+`Superseded by ADR-NNNN`; no others (an ADR is never `Approved`). Status
+changes (`Proposed` → `Accepted` → `Deprecated` / `Superseded by ADR-NNNN`, or
+`Proposed` → `Rejected`) are new rows in `## Version History` (only the two
+latest are kept; earlier ones stay in git); never rewrite a retained row.