feat(gh): prompt for repository when not in a git repo

[YousefHadder]

Closes: #2565

When running Snacks.picker.gh_issue() or Snacks.picker.gh_pr() outside of a git repository, the user is now prompted to enter a repository in owner/repo format.

Changes

• Added gh_prompt_repo helper function that prompts for repository input
• Added abort config option to allow aborting picker creation
• Updated documentation with new feature and usage examples

Usage

When not in a git repo:

Snacks.picker.gh_issue()  -- prompts for repo
Snacks.picker.gh_pr()     -- prompts for repo

-- Or specify directly:
Snacks.picker.gh_issue({ repo = "folke/snacks.nvim" })

Screenshots

While not in a local repo: