refactor(github)!: deprecate legacy hub plugin - #14089

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• If I used AI tools (ChatGPT, Claude, Gemini, etc.) to assist with this contribution, I've disclosed it below.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• This change does not introduce new aliases.

Changes:

• Deprecate the github plugin, which supports the legacy hub CLI, and direct GitHub CLI users to the gh plugin.
• Keep existing hub behavior during the deprecation period while making the repository helpers use the current branch and stop after a failed directory change.
• Replace stale github documentation with migration guidance for gh repo create.
• Preserve the last valid gh completion cache when completion generation fails.
• Correct the documented gh completion cache location.

Testing:

• Ran zsh -n against both plugin files.
• Ran git diff --check.
• Loaded both plugins through oh-my-zsh.sh.
• Generated and loaded completion using GitHub CLI v2.100.0.
• Simulated failed completion generation and verified the existing cache remained unchanged.
• Verified exist_gh does not call hub after a failed cd and pushes HEAD for a valid directory.

Other comments:

GitHub CLI is not a drop-in Git wrapper, so this intentionally does not replace alias git=hub with gh. Existing hub users retain the old behavior until the deprecated plugin is removed.

AI-assisted: OpenCode assisted with research, implementation, and test setup; the resulting changes were reviewed and tested manually.