* fix(metrics): remove productivity score drop when focus passes target
The focus-progress soft-cap clamped its input to [0,1], so any focus
beyond the target collapsed to a constant 0.889 credit: with impact 3,
240 min of focus scored 81 while 241 min scored 77, and the score never
recovered no matter how much more focus was logged. Focus progress is
now linear to the target and flat beyond it - continuous and
non-decreasing - leaving overwork penalties to the sustainability score.
Also removes the dead workloadLinearZeroAt parameter from
calculateSustainabilityScore (ignored since the sigmoid rewrite) and its
misleading call-site comments, renames DEFAULT_WORKLOAD_LINEAR_ZERO_AT
to DEFAULT_MAX_WORK_MINUTES to match what it does, and fixes stale
JSDoc (removed exhaustion param, outdated example values).
* fix(metrics): preserve impact mismatch classification
* docs(metrics): clarify productivity focus credit
