PR: Fix gh repo rename owner-prefix UX confusion

Summary

Fixes #13034.

When gh repo rename prompts interactively, the prompt shows Rename owner/repo to: - a format
that naturally leads users to type owner/new-name. The command previously rejected any name
containing / with an error message about repository transfers, which is unrelated to what the
user was trying to do.

This PR makes three focused changes to pkg/cmd/repo/rename/rename.go:

• Reword the prompt to New name for owner/repo (without owner prefix): - removes the ambiguity
  before the user types anything.
• Strip the same-owner prefix automatically when the user types owner/new-name and owner
  matches the current repository's owner. A warning is printed to stderr so the user sees what
  happened.
• Improve the error for the different-owner case - the new message explains what format is
  expected, and reserves the transfer doc link for when it's actually relevant (a different owner
  was typed).

Behavior change

Same-owner prefix (new: strips and proceeds):

$ 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/new-name

Different-owner prefix (new: 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>

Non-interactive path (gh repo rename new-name) -unchanged.

Test plan

• All existing tests updated to match the new prompt text - all pass.
• error on different-owner prefix: verifies the new error message for a mismatched-owner input.
• strips same-owner prefix: verifies that OWNER/NEW_REPO input against an OWNER/REPO repo
  succeeds and calls the API with NEW_REPO.

go test ./pkg/cmd/repo/rename/... -v

Notes

• Owner comparison uses strings.EqualFold -GitHub usernames are case-insensitive.
• strings.SplitN(..., 2) limits splitting to one /, so inputs like owner/a/b produce
  parts[1] = "a/b" which still contains /. This hits the same-owner-strip path, strips the
  prefix to "a/b", and then the API rejects it with a server-side error - correct behavior,
  no special casing needed.
• Related prior work: gh repo rename help text clarifies new repo name should not include owner #10044 (help text), Error when gh repo rename is used with a new repo name that contains an owner #10364 (slash validation added).