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/templates/OC.md

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+# @TITLE@
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | @ID@ |
+| CrossReference | @CROSSREF@ |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| @DATE@ | Accepted | @AUTHOR@ | <reviewer S-ID> | Initial version | pending |
+
+---
+
+## Contract: <operationName>
+
+| Item | Value |
+| --- | --- |
+| Operation | `operationName(param: Type): ReturnType` |
+| Traces to | <SSD message> in [SSD-<n>] |
+| Domain Model concepts | <Concept, Association> ([DM-<n>]) |
+
+**Preconditions**
+
+- <required state in Domain Model terms>
+
+**Postconditions**
+
+- A <Concept> instance was created.
+- <Concept> was associated with <Concept>.
+- <Concept>.<attribute> was set to <value>.
+
+**Exceptions**
+
+| Condition (failing precondition) | Outcome |
+| --- | --- |
+
+---
+
+@LINKS@