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/OC.md

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+# Operation Contract (OC)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **SSD** — **Required source**: each contract traces to exactly one SSD message
+- **DM** — Classes/associations named in pre/postconditions
+- **SD** — Forward: the design realizing each contract's postconditions
+
+## Required sections (after Metadata / Version History)
+
+One block per system operation, each containing:
+
+1. **Operation** — complete signature: name, parameter types, return type
+   (must match the SSD message).
+2. **Cross References** — the SSD message it traces to (one contract per
+   message) and the Domain Model concepts touched.
+3. **Preconditions** — required state before execution, expressed in Domain
+   Model terms.
+4. **Postconditions** — state changes only, in Larman's style: *instance
+   created / instance associated / attribute modified*. Declarative ("what"),
+   never algorithmic ("how"); avoid vague text like "system processes the
+   request".
+5. **Exceptions** — error conditions, each with the failing precondition.
+
+## Terminology
+
+Use the IT terms from the dictionary (`DICT`), not the PO terms the Domain Model uses.