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/UC.md

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+# Use Case (UC)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **UCD** — Actor and use-case names must match it exactly
+- **US** — Stories this use case decomposes into
+- **SA** — Stakeholders & Interests — cite S-IDs
+- **DM** — Forward: concepts derived from this use case's nouns
+- **SSD** — Forward: the SSD depicting this use case's main scenario
+
+## Location
+
+`docs/uc-NNN/uc.md`, one folder per use case (create with
+`new-artifact.sh UC --file docs/uc-NNN/uc.md`). The artifacts this use case
+affects are saved in the same folder; see the `project-planning` skill, "Use
+cases get their own folder".
+
+## Required sections (after Metadata / Version History)
+
+Pick the format explicitly (`Format: Brief | Casual | Fully Dressed`) and
+state the **scope/level** (summary, user-goal, subfunction). Always: primary
+actor, pre/postconditions, goal-perspective wording with no UI or
+implementation detail.
+
+- **Brief** — a single paragraph summarizing only the main success scenario.
+- **Casual** — informal multi-paragraph narrative; may mention some
+  alternate flows.
+- **Fully Dressed** — all sections, in order: Scope, Level, Primary Actor,
+  Stakeholders and Interests (cite `SA` S-IDs), Preconditions,
+  Postconditions (success guarantee), Main Success Scenario (numbered
+  steps), Extensions / Alternative Flows (reference `<<include>>` /
+  `<<extend>>` use cases), Special Requirements / Business Rules (per step),
+  Open Issues.
+
+Title and actor names must match `UCD` and `US` exactly.