ci(release): fallback to direct merge when auto-merge is disabled

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Summary

Fixes action=full release orchestration so it no longer fails when repository-level GitHub auto-merge is disabled.

What changed

• Updated Release / Sure-AIO full-release merge step to:
  • try gh pr merge --auto first
  • detect enablePullRequestAutoMerge limitation
  • automatically fall back to direct merge polling/retry
• Preserved one-dispatch release flow: prepare PR -> merge -> tag/release -> dispatch image publish workflow.
• Updated release documentation to reflect fallback behavior.

Why

action=full previously failed in repos where GitHub auto-merge is not enabled, causing release orchestration to stop mid-flow and creating operational churn.

Validation

• Parsed workflow YAML successfully (release.yml ok)
• Verified fallback logic paths and docs updates
• Confirmed branch pushed and ready for PR

Notes

• After merge, action=full should work in both environments:
  • auto-merge enabled
  • auto-merge disabled (fallback path)