fix(squads): warn leader against double-triggering an agent

Squad coordinators were both @mentioning an agent in the parent issue and
creating a todo child issue assigned to the same agent, causing the agent
to be triggered twice in parallel (mention dispatch + assignment dispatch).
The server has no cross-issue dedupe for this case — and adding one would
make @mention semantics context-dependent and unpredictable.

Fix is at the prompt level: tell the squad leader that a `todo` child
issue with an agent assignee already fires that agent, so they must pick
exactly one delegation path for any given piece of work — comment-based
@mention or todo child-issue assignment, never both.

Adds a focused regression test that locks in the new rule via narrow
substring checks (so harmless rewording stays free).

Fixes #3033

Co-authored-by: multica-agent <github@multica.ai>

server/internal/handler/squad_briefing.go

@@ -79,7 +79,14 @@ Hard rules:
   explaining the gap (and @mention the issue's reporter if possible)
   rather than silently doing the work.
 - ALWAYS call ` + "`" + `multica squad activity` + "`" + ` before ending your turn —
-  even when the outcome is no_action.`
+  even when the outcome is no_action.
+- A child issue you create with ` + "`" + `--status todo` + "`" + ` and an agent assignee
+  already fires that agent automatically — the assignment IS the trigger.
+  If you also @mention the same agent on this parent issue for the same
+  work, the agent runs twice in parallel (once from the mention, once
+  from the assignment). Pick exactly one path: either delegate by
+  @mention on this issue, or create a ` + "`" + `todo` + "`" + ` child issue assigned to
+  them. Never both for the same work.`
 
 // buildSquadLeaderBriefing composes the full system briefing appended to a
 // squad leader's Instructions when it claims a task on a squad-assigned

server/internal/handler/squad_briefing_test.go

@@ -13,6 +13,24 @@ import (
 	db "github.com/multica-ai/multica/server/pkg/db/generated"
 )
 
+// TestSquadOperatingProtocolWarnsAgainstDualTrigger locks in the rule
+// added for #3033: the protocol must tell the squad leader that a `todo`
+// child issue with an agent assignee already fires that agent, so they
+// must not also @mention the same agent on the parent issue for the
+// same work. Asserts behavior, not exact wording — keep the substrings
+// narrow so harmless rewording doesn't break the test.
+func TestSquadOperatingProtocolWarnsAgainstDualTrigger(t *testing.T) {
+	compact := strings.Join(strings.Fields(squadOperatingProtocol), " ")
+	for _, want := range []string{
+		"--status todo` and an agent assignee already fires that agent automatically",
+		"Never both for the same work.",
+	} {
+		if !strings.Contains(compact, want) {
+			t.Errorf("expected squad operating protocol to contain %q\n--- protocol ---\n%s", want, squadOperatingProtocol)
+		}
+	}
+}
+
 // seedSquadForBriefing creates a squad with the seeded test agent as
 // leader. Returns the loaded db.Squad and a cleanup-registered ID.
 func seedSquadForBriefing(t *testing.T, leaderID string, name, instructions string) db.Squad {