Remove hardcoded sidechat UUIDs from dm.py (heartbeat, 646-opm-work)

Thread UUIDs rotate -- heartbeat-opm died twice in one day
(5bd5b350 -> 0077e918 -> dead), dropping self_main_loop digests.
SIDCHAT_ALIASES is now intentionally empty; targets fall through to
job-sidechats.json dynamic mappings then name-based sidechat use with
autoprovision, which self-heals. Also removed stale heartbeat-opm and
pipe-9735f2 entries (dead UUID 0077e918) from job-sidechats.json.

Test: dm.py send to heartbeat resolved via name and SENT+VERIFIED
(thread 5f18476d-8994-49e7-a9e0-4838732363fe).

Session: sidechat/chromebox-ops

job-sidechats.json

@@ -1,5 +1,4 @@
 {
-  "heartbeat-opm": "0077e918-9ac8-40ba-b0ad-65f9f78037c4",
   "646-pip-coord": {
     "thread_uuid": "4466d0c1-7961-4cf3-b99d-1ab7c38484c2",
     "agent": "pip"
@@ -18,11 +17,6 @@
     "agent": "opm",
     "created_at": "2026-10-04T16:39:34.721269+00:00"
   },
-  "pipe-9735f2": {
-    "thread_uuid": "0077e918-9ac8-40ba-b0ad-65f9f78037c4",
-    "agent": "opm",
-    "created_at": "2026-10-04T16:43:03.587214+00:00"
-  },
   "pipe-7d2896": {
     "thread_uuid": "78e03d7a-0ba2-440e-a730-6f3b3737cbac",
     "agent": "opm",