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/TM.md

@@ -0,0 +1,30 @@
+# @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 |
+
+---
+
+## Purpose
+
+Tracks backward/forward links between artifact instances so that the Business Case's
+cross-artifact traceability success criterion is measurable. A row is added or
+updated whenever an artifact instance is created or reviewed.
+
+## Traceability Table
+
+| Artifact Instance | Type | Upstream (Backward Link) | Downstream (Forward Link) | Last Reviewed (RC-ID) |
+| --- | --- | --- | --- | --- |
+| [<ID>] | <type> | [<upstream-ID>] | [<downstream-ID>] | [<RC-ID>] |
+
+---
+
+@LINKS@