Problem: picker backend names were hard-coded as string literals in
`picker/init.lua`, `health.lua`, and `init.lua` validation, making
them easy to get out of sync.
Solution: export `backends` and `detect_order` from `forge.picker` and
reference them in health check and config validation instead of
duplicating the name strings.
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.