planning: add issue migrate-closed-issues-convention to 2.1

update conventions and all migration scripts to include closed issues in planning file format migrations

Author: cowwoc

.cat/issues/v2/v2.1/migrate-closed-issues-convention/index.json

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+{"status": "open"}

.cat/issues/v2/v2.1/migrate-closed-issues-convention/plan.md

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+# Plan
+
+## Goal
+
+Update the conventions to specify that even closed issues should be migrated when updating
+the format of planning files. Update all migration scripts (1.0.8.sh, 1.0.9.sh, 1.0.10.sh,
+2.0.sh, 2.1.sh) to process closed issues. Update all relevant convention files to reflect
+the new policy.
+
+## Pre-conditions
+
+(none)
+
+## Post-conditions
+
+- [ ] User-visible behavior unchanged (existing open-issue migration continues to work correctly)
+- [ ] All migration scripts updated to process both open and closed issues
+- [ ] Migration scripts remain idempotent (running twice produces no additional changes)
+- [ ] CLAUDE.md and all relevant convention files updated to reflect the new policy
+- [ ] Tests passing
+- [ ] E2E: Run an updated migration script on a repo with closed issues and verify they are migrated