fix(session): stop training the model to parrot [Message Time: ...]

Past assistant turns in history were prefixed with "[Message Time: ...]"
just like user turns. The model treated these as in-context demos and
started prefixing its own replies with the same marker, leaking
metadata to the user. Prompt-level warnings could not beat dozens of
prior assistant samples.

Annotate only user turns and proactive deliveries
(_channel_delivery=True, i.e. cron / heartbeat pushes whose timing is
the whole point and which are too infrequent to act as demos). Adjacent
user-side timestamps still pin every normal assistant reply for
relative-time reasoning. The now-redundant identity.md warning is
removed along with the demonstration source.

nanobot/templates/agent/identity.md

@@ -28,9 +28,5 @@ Output is rendered in a terminal. Avoid markdown headings and tables. Use plain
 - On broad searches, use `grep(output_mode="count")` to scope before requesting full content.
 {% include 'agent/_snippets/untrusted_content.md' %}
 
-Historical messages may include `[Message Time: ...]` prefixes. Treat them as
-metadata for chronology only; never quote, copy, or include those markers in
-your response.
-
 Reply directly with text for conversations. Only use the 'message' tool to send to a specific chat channel.
 IMPORTANT: To send files (images, video, audio, documents) to the user, you MUST call the 'message' tool with the 'media' parameter. Do NOT use read_file to "send" a file — reading a file only shows its content to you, it does NOT deliver the file to the user. Examples: message(content="Here is the image", media=["/path/to/file.png"]) or message(content="Here is the video", media=["/path/to/video.mp4"])