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.

website/docs/user-guide/messaging/index.md

@@ -327,6 +327,24 @@ display:
   tool_progress_grouping: accumulate   # accumulate | separate
 ```
 
+### Message timestamps in model context
+
+Off by default. When enabled, Hermes prepends a human-readable timestamp
+(e.g. `[Tue 2026-04-28 13:40:53 CEST]`) onto each **user** message *in the
+model's context* so the agent knows when messages were sent — useful for
+temporal reasoning ("you asked this morning…", noticing a long gap). It is
+**not** added to assistant messages or the system prompt.
+
+```yaml
+gateway:
+  message_timestamps:
+    enabled: false   # set true to show send-times to the model
+```
+
+Persisted transcripts always stay clean — the timestamp is stored as message
+metadata regardless of this toggle, so enabling it later also surfaces
+send-times for past messages, and replay never accumulates duplicate prefixes.
+
 When enabled, the bot sends status messages as it works:
 
 ```text