feat: require evidence to complete and a concrete blocker to block

A [goal:complete] is now only archived when accompanied by a non-empty
[goal:evidence] line, and a [goal:blocked] only stops the goal when a
concrete blocker is stated. Unsubstantiated claims are rejected and the
plugin re-prompts for the missing evidence/blocker via a new
<evidence_required> continuation block. Evidence is stored on the result
and surfaced in /goal status and /goal history.

Adds extractCompletionEvidence, threads evidence through result
persistence, updates continuation/system/compaction/creation prompts and
the README, and adds unit + handler tests. Implements megalist item 2.1.