fix(ci): make downstream bumps retry-safe

[codeforester]

Summary

• Deduplicate the stable marker across all downstream issues and pull requests.
• Reopen and reuse an existing marked issue when a previous run stopped before PR publication.
• Comment actionable failure guidance on a preserved issue so retries can repair partial state.
• Document the retry policy and retain the no-diff close path.

Issue

Fixes #2189

Validation

• bash -n .github/workflows/downstream-version-bumps.yml
• env -u BASE_HOME BASE_CACHE_DIR=/private/tmp/base-issue-2189-cache PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest -q tests/test_github_workflows.py tests/test_downstream_version_bump.py
• 46 passed
• git diff --check

Docs Impact

Documented the cross-issue/PR marker lookup and preserved-issue retry policy in the release process.