fix(engine,core): dedup heartbeat-spawned follow-ups by parent task

Same-agent intake guard now also matches siblings sharing a
sourceParentTaskId, so repeated heartbeats from one parent task
can't bypass dedup just because triage rewrites the title.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/parent-task-dedup-intake.md

@@ -0,0 +1,22 @@
+---
+"@fusion/core": minor
+"@fusion/engine": minor
+---
+
+fix(engine,core): dedup heartbeat-spawned follow-ups by parent task
+
+Heartbeat agents create follow-up tasks via `fn_task_create`. Until
+now, the intake similarity guard scoped candidates by `sourceAgentId`
+only, so the same parent task could spawn many sibling tasks across
+heartbeats whenever triage rewrote their titles enough to dodge the
+title-fingerprint guard.
+
+The task-scoped heartbeat now stamps `sourceParentTaskId` (and
+`sourceRunId`) on every `fn_task_create`, and the intake duplicate
+matcher treats a candidate as a sibling when it shares either the
+caller's agent ID or the caller's parent task ID. Same-parent
+siblings with similar descriptions are auto-archived as before.
+
+Tool description and heartbeat prompts also now instruct agents to
+scan existing open tasks before creating, as a belt-and-suspenders
+layer above the deterministic dedup.