All 27 sentinel error variables across 6 packages used fmt.Errorf with
static strings (no format verbs). Replaced with errors.New which is
semantically correct and avoids unnecessary fmt import in 5 of 6 files.
Only pool.ErrReasonTooLong retains fmt.Errorf (uses %d format verb).
Packages: pool, rpi, ratchet, vibecheck, storage, types.
Move 9 sentinel error vars from pool.go into a dedicated errors.go
file, matching the pattern used in ratchet, storage, vibecheck, rpi,
and types packages. All 6 packages now have errors.go.
Boden Fuller