Obsolete tasks

A task can be obsolete when it is a duplicate, superseded or no longer needed.

mdboard obsolete '#0925-05' --reason "Duplicate of #0925-03"
mdboard reopen-task '#0925-05' --reason "Required again"

• Task-only terminal state.
• Reason required.
• Allowed from pending, working or error.
• Obsolete is resolved but not delivered.
• Completion percentages count only done.
• Handover accepts done or obsolete, with at least one done.
• canceled still blocks handover.
• Obsolete tasks are skipped by next.
• HTML/JSON show the state; batch status reports a separate count.