Problem: `|:checkhealth|` is not a valid vimdoc tag, causing the
vimdoc-language-server check to fail in CI.
Solution: use inline code (\`:checkhealth forge\`) instead of a tag
reference.
Problem: docs listed fzf-lua as the only required dependency, and the
health check only verified fzf-lua.
Solution: update requirements to list all three picker backends, add
`picker` config option to vimdoc, update health check to report all
detected backends and mark the active one.
Problem: vimdoc did not document per-forge feature differences or
the new capabilities field on the forge interface.
Solution: add COMPATIBILITY section with per-forge feature matrix,
document `capabilities` in the `forge.Forge` interface fields, and
update the source skeleton example.
