fix(upgrade): show safe shutdown progress

[ScriptedAlchemy]

Summary

• tell users when update/upgrade enters the clean shutdown window
• explicitly warn not to interrupt while writers drain
• confirm exclusive maintenance access before mutations

Verification

• affected lifecycle tests: 18 passed
• cargo test-all --status-level fail --final-status-level fail: 4665 passed, 2 skipped
• cargo dogfood: update shutdown message and daemon restart verified
• tracedecay doctor: database integrity ok, no issues

[changeset-bot]

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Latest commit: e008fd9

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