CLI command to stage deletions from search results

[michael-dm]

Currently the only way to stage messages for deletion is through the TUI (select → d). It would be useful to have a CLI command that stages deletions from search results, e.g.:

msgvault stage-delete "from:newsletter@example.com older_than:1y"
msgvault stage-delete --ids 123,456,789

This would enable:

• Scripted/automated cleanup (e.g. in cron jobs chained with delete-staged --trash --yes)
• Bulk operations on large result sets without manually selecting in the TUI (related: TUI: how to bulk delete after searching? #41)
• Integration with AI agents that can search and triage but can't drive the TUI

The existing delete-staged execution flow (stage → review → execute) would stay intact — this just adds a non-interactive way to create the staging manifest.