chore(agents): improve architecture-pool-supervisor — add milestone assignment to spec PRs

Approved proposal: #7521
Pattern: workflow_fix
Evidence: Watchdog (Cycle 13, #7511) consistently flags 8 spec PRs missing
milestones: #7362, #7376, #7378, #6908, #6745, #6739, #6729, #6723. All
created by the architecture agent without milestone assignment.
Fix: Added step 5 to PR workflow requiring milestone assignment via
forgejo_update_pull_request after PR creation.

ISSUES CLOSED: #7521

.opencode/agents/architecture-pool-supervisor.md

@@ -398,9 +398,15 @@ When making major changes to an existing spec:
    - Body: comprehensive description of every architectural change, the
      rationale, alternatives considered, and any open questions.
 4. Add the label **`needs feedback`** to the PR.
-5. **Do NOT merge this PR.** A human must review and merge it.
-6. Post a comment on the session state issue noting the PR.
-7. Return the PR number so the product-builder can monitor it and continue
+5. **Assign the PR to the current active milestone** using `forgejo_update_pull_request`:
+   - Query the current active milestone via `forgejo_list_repo_milestones` (state=open)
+   - Select the earliest open milestone that this spec change addresses
+   - Call `forgejo_update_pull_request` with `milestone=<milestone_id>`
+   - If no clear milestone applies, skip this step rather than guessing
+   - This ensures spec PRs are traceable to the milestone they support
+6. **Do NOT merge this PR.** A human must review and merge it.
+7. Post a comment on the session state issue noting the PR.
+8. Return the PR number so the product-builder can monitor it and continue
    other work while waiting for human approval.
 
 The system does NOT block on spec PR approval. It continues implementing