The triage class validator in agent-send.sh rejected `digest`, the
canonical machine-wake / coalescible message class defined by the
converged wake/heartbeat design (CONVERGED-DESIGN §2.3 sol C1). Add
`digest` to the accepted class set (case-statement validator + the
documented consumer grammar regex), alongside the existing
terminal-log/actionable/human/reaction classes.
Red-first: extended agent-send.test.sh with --class digest / -C digest
assertions plus a grammar-regex case for class=digest. At origin/main
these 3 new assertions FAIL (PASS=15 FAIL=3); with the validator fix
applied, all 18 assertions PASS (PASS=18 FAIL=0). All pre-existing
assertions were unaffected throughout.
Part of #892
