slot targeting follow-ups from #7630

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