fix(new-milestone): add explicit MILESTONE-CONTEXT.md reference

The context section now includes @.planning/MILESTONE-CONTEXT.md so
Claude actually loads the file from discuss-milestone instead of just
seeing a text mention in the process steps.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/gsd/new-milestone.md

@@ -30,6 +30,9 @@ Milestone name: $ARGUMENTS (optional - will prompt if not provided)
 @.planning/STATE.md
 @.planning/MILESTONES.md
 @.planning/config.json
+
+**Load milestone context (if exists, from /gsd:discuss-milestone):**
+@.planning/MILESTONE-CONTEXT.md
 </context>
 
 <process>