Problem: Forge refs required an issue/PR number (`gh:user/repo#42`).
Users wanting to link a repo without a specific issue had no option.
Solution: Accept `gh:user/repo` shorthand and `https://github.com/user/repo`
URLs as `type='repo'` refs with `number=nil`. These conceal and render
virtual text like numbered refs but skip all API calls (no validate, no
fetch, no close). `format_label` strips `#%n` for bare refs. Omnifunc
offers both `owner/repo#` and `owner/repo` completions.
Closes#135
287a4e32e3