refactor(cli): drop CLAUDEMESH_SESSION_PRESENCE flag

per-session presence is small and uncomplicated enough that a rollback
flag isn't load-bearing. backwards compat is already covered at the
protocol layer — older brokers reply unknown_message_type to
session_hello and the SessionBrokerClient marks itself closed for that
mesh, which is the same outcome the flag would have given. removing
the flag, the helper, and the conditional from the registry hook.

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

apps/cli/CHANGELOG.md

@@ -31,9 +31,12 @@ access shape).
   `parent_attestation`). Older payloads continue to work.
 - **`claudemesh launch`** — generates an ed25519 session keypair and a
   12 h parent attestation per launch (mesh secret key signs it),
-  forwards both to the daemon under `body.presence`. The flag
-  `CLAUDEMESH_SESSION_PRESENCE` defaults to ON; set `=0` to roll back
-  if a broker on a given mesh is misbehaving.
+  forwards both to the daemon under `body.presence`. Per-session
+  presence is always on; older brokers that don't recognize
+  `session_hello` reply `unknown_message_type` and the daemon quietly
+  drops the per-session WS for that mesh — the regular member-keyed
+  WS still covers all functionality, the only loss is sibling-session
+  visibility on that mesh.
 - **latent 1.29.0 bug fix** — `claudemesh launch` referenced
   `claudeSessionId` before its `const` declaration further down the
   file, hitting the temporal dead zone → `ReferenceError` silently
@@ -44,10 +47,9 @@ access shape).
 
 ### Sequencing
 
-The broker side ships first and bakes for ~24 h. Only then does the
-flag default flip on the CLI side. Older CLIs continue working
-unchanged (no per-session WS), and the protocol is purely additive on
-the wire.
+The broker side ships first and bakes for ~24 h. Older CLIs continue
+working unchanged (no per-session WS), and the protocol is purely
+additive on the wire.
 
 ### Verification (smoke)