docs(5288): externalise subagent brief, make property mandatory
- Move the subagent brief out of tasks.md into specs/008-script-witness-parity/briefs/T010-T013.md so the task list stays scannable. tasks.md now points at the brief rather than embedding it. - Add T013 (US3): one QuickCheck property prop_buildLedgerTx_matches_mkUnsignedTx_on_script_witnesses over random (SelectionOf TxOut, ScriptWitnesses) pairs within documented generator bounds. The property is acceptance-gating; withMaxSuccess 100; same equality assertions as the six enumerated cases. - Revise research.md D6 (property was "optional"; now mandatory) and plan.md "Tests" (was "optional/deferred"; now mandatory) so all artifacts agree.