feat(gateway): gate message timestamps behind opt-in (default off)

Follow-up to salvaged PR #41633: the timestamp prefix injection was
unconditional. Gate the in-context render behind
gateway.message_timestamps.enabled (default false) at both the live-message
and history-replay sites; timestamp metadata is still captured + persisted
regardless so the toggle can be flipped on later. Add DEFAULT_CONFIG entry,
docs, and gate tests.

hermes_cli/config.py

@@ -2270,6 +2270,17 @@ DEFAULT_CONFIG = {
     # Gateway settings — control how messaging platforms (Telegram, Discord,
     # Slack, etc.) deliver agent-produced files as native attachments.
     "gateway": {
+        # Inject a human-readable timestamp prefix (e.g.
+        # "[Tue 2026-04-28 13:40:53 CEST]") onto user messages IN THE MODEL'S
+        # CONTEXT so the agent has temporal awareness of when each message was
+        # sent. Off by default — when off, the model sees clean message text.
+        # Persisted transcripts always stay clean (the timestamp is stored as
+        # message metadata regardless of this toggle), so turning it on later
+        # surfaces send-times for past messages too.
+        "message_timestamps": {
+            "enabled": False,
+        },
+
         # When false (default), any file path the agent emits is delivered
         # as a native attachment as long as it isn't under the credential /
         # system-path denylist (/etc, /proc, ~/.ssh, ~/.aws, ~/.hermes/.env,