feat(framework): accept digest message class in agent-send.sh (W1)

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

packages/mosaic/framework/tools/tmux/agent-send.sh

@@ -46,6 +46,7 @@
 #                     actionable    carries a decision/blocker/gate — deliver
 #                     human         from a human operator — deliver
 #                     reaction      an emoji/ack reaction
+#                     digest        machine-wake, coalescible; batched wake/heartbeat signal
 #                   Long form: --class CLASS (or --class=CLASS). When SET, the
 #                   preamble carries a ` class=<CLASS>` token INSIDE the bracket:
 #                       [<src> -> <dst> class=terminal-log] <message>
@@ -58,7 +59,7 @@
 #   -h              help
 #
 # PREAMBLE GRAMMAR  (for consumers / daemons mirroring this producer)
-#   ^\[(\S+) -> (\S+?)(?: class=(terminal-log|actionable|human|reaction))?\] (.*)$
+#   ^\[(\S+) -> (\S+?)(?: class=(terminal-log|actionable|human|reaction|digest))?\] (.*)$
 #     group 1 = src label   group 2 = dst host:session
 #     group 3 = class (absent => actionable)   group 4 = message body
 #
@@ -109,8 +110,8 @@ done
 CLASS_TOKEN=""
 if [ -n "$CLASS" ]; then
   case "$CLASS" in
-    terminal-log|actionable|human|reaction) CLASS_TOKEN=" class=${CLASS}" ;;
-    *) echo "ERROR: invalid --class '$CLASS' (allowed: terminal-log, actionable, human, reaction)" >&2; exit 3 ;;
+    terminal-log|actionable|human|reaction|digest) CLASS_TOKEN=" class=${CLASS}" ;;
+    *) echo "ERROR: invalid --class '$CLASS' (allowed: terminal-log, actionable, human, reaction, digest)" >&2; exit 3 ;;
   esac
 fi