slot targeting follow-ups from #7630

[rajeshkamal5050]

Minor items from PR #7630 review, none blocking.

1. case-sensitive swap vs case-insensitive deploy — swap's isValidSlotName uses slices.Contains (exact match), deploy uses strings.EqualFold. Align swap with slices.ContainsFunc + strings.EqualFold.

2. @production undocumented — normalizeSlotName accepts @production but flag help only mentions production. Add a note.

3. deploy loop over single-element slice — determineDeploymentTargets now always returns one target. Simplify the loop in Deploy().

4. add case-insensitive slot name test — have test for Production → main app, but not Staging → staging slot. Add one.

Ref: #7365, #7630