docs: add Gitea milestone tracking instructions to all phase docs

Each phase doc now includes:
- Milestone Tracking section linking to the corresponding Gitea milestone
- Checklist item in Completion Criteria to close the milestone
- Phase 7 also closes the Final: Porting Complete milestone (ID 8)

docs/plans/phases/phase-3-library-mapping.md

@@ -20,6 +20,10 @@ returns an empty list and every import has a documented .NET migration path.
 | Go source code | `golang/` (specifically `golang/nats-server/`) |
 | .NET ported version | `dotnet/` |
 
+## Milestone Tracking
+
+This phase corresponds to **Milestone 3** in [Gitea](https://gitea.dohertylan.com/dohertj2/natsnet/milestone/3). When starting this phase, verify the milestone is open. Assign relevant issues to this milestone as work progresses.
+
 ## Steps
 
 ### Step 1: List all unmapped libraries
@@ -252,6 +256,14 @@ Phase 3 is complete when ALL of the following are true:
       guidance).
 - [ ] `phase check 1` shows all items checked including "All libraries mapped".
 - [ ] A mapping report has been exported for reference.
+- [ ] Close the Phase 3 milestone in Gitea:
+  ```bash
+  curl -s -X PATCH "https://gitea.dohertylan.com/api/v1/repos/dohertj2/natsnet/milestones/3" \
+    -H "Content-Type: application/json" \
+    -H "Authorization: token $GITEA_TOKEN" \
+    -d '{"state":"closed"}'
+  ```
+  Or close it via the Gitea web UI at https://gitea.dohertylan.com/dohertj2/natsnet/milestone/3
 
 ## Troubleshooting