test(go): expand coverage for repl and config

[halukerenozlu]

What does this PR do?

Expands test coverage for REPL command behavior, config secret-preserving saves, and modal widget interactions. The change is test-only and does not modify runtime behavior.

Related issues

N/A

Changes

• Add CommandRegistry.All() ordering coverage
• Add REPL tests for skill listing/details, /use, model switching, and active provider/model helpers
• Add config tests for SaveFilePreservingSecrets
• Add modal widget tests for close/no-match behavior, cursor bounds, input values, and item rendering

How to test

Steps a reviewer can follow to verify the change manually:

1. Run go test ./... -count=1
2. Run go test -coverprofile .\coverage.out ./...
3. Run go tool cover -func .\coverage.out and confirm total coverage is around 68.7%

Checklist

• Tests pass (go test ./... -count=1)
• No vet warnings (go vet ./...)
• Build succeeds (make build)
• Conventional commit message (e.g. feat(go/agent): ...)
• Docs updated if behaviour changed
• No breaking changes (or described below)