gh repo rename interactive prompt causes owner-prefix confusion

Description

When renaming a repository interactively, the prompt shows the current repository's full name
(owner/repo) in a format that naturally leads users to type the new name in the same format
(owner/new-name). However, the command only accepts the bare name (new-name).

The resulting error message is misleading: it warns about transferring the repository to a new
owner, which is unrelated to what the user was trying to do.

Steps to reproduce

$ gh repo rename
Rename digitalby/raycast-input-layout-switch-poc to: digitalby/raycast-input-layout-switch
error: New repository name cannot contain '/' character - to transfer a repository to a new owner,
see <https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>.

Expected behavior

One of:

• The prompt should hint at the expected format, so users don't type the owner prefix in the
  first place.
• Or, if the user types their own owner prefix, the command should strip it and proceed - since
  the intent is unambiguous.
• The error message, when shown, should explain what to type instead of pointing to a
  repository-transfer doc page.

Actual behavior

The user gets an error that talks about repository transfers, with no indication of what the
correct input format is.

Proposed fix

Three-part change to pkg/cmd/repo/rename/rename.go:

1. Reword the prompt

// Before
opts.Prompter.Input(fmt.Sprintf("Rename %s to:", ghrepo.FullName(currRepo)), "")

// After
opts.Prompter.Input(fmt.Sprintf("New name for %s (without owner prefix):", ghrepo.FullName(currRepo)), "")

2. Smart-strip the same-owner prefix instead of rejecting it

// Before
if strings.Contains(newRepoName, "/") {
    return fmt.Errorf("New repository name cannot contain '/' character - ...")
}

// After
if strings.Contains(newRepoName, "/") {
    parts := strings.SplitN(newRepoName, "/", 2)
    if strings.EqualFold(parts[0], currRepo.RepoOwner()) {
        // User typed their own owner prefix - strip it and proceed
        newRepoName = parts[1]
        fmt.Fprintf(opts.IO.ErrOut, "! Owner prefix %q stripped - renaming to %q\n", parts[0], newRepoName)
    } else {
        return fmt.Errorf(
            "to rename, enter only the new repository name without an owner prefix.\n" +
            "To transfer this repository to a different owner, visit GitHub.com:\n" +
            "<https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>",
        )
    }
}

3. Result

Same-owner prefix:

$ gh repo rename
New name for digitalby/poc (without owner prefix): digitalby/new-name
! Owner prefix "digitalby/" stripped - renaming to "new-name"
Renamed repository digitalby/poc to digitalby/new-name

Different-owner prefix (clear, actionable error):

$ gh repo rename
New name for digitalby/poc (without owner prefix): otherorg/new-name
error: to rename, enter only the new repository name without an owner prefix.
To transfer this repository to a different owner, visit GitHub.com:
<https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>

Notes

• The non-interactive path (gh repo rename new-name) is unaffected - it works correctly today.
• The same validation block handles both interactive and argument-provided names, so the fix
  covers both paths.
• Owner comparison uses case-insensitive matching (strings.EqualFold) since GitHub usernames
  are case-insensitive.
• This was previously partially addressed by PR gh repo rename help text clarifies new repo name should not include owner #10044 (help text) and PR Error when gh repo rename is used with a new repo name that contains an owner #10364 (slash validation),
  but the interactive UX gap remains.