messages: unify harness-injected data under <system-extra> tag

Replace the ad-hoc [SYSTEM INFO] / [TELEGRAM SYSTEM INFO] prefixes with a
single canonical <system-extra> wrapper, sourced from one constant
(SYSTEM_EXTRA_TAG) so emission and documentation can never diverge.

- core-api: SYSTEM_EXTRA_TAG + system_extra() helper; attachments_block
  rebuilt on top of it.
- telegram: system_info_message (location) uses the helper; the voice
  transcript is forwarded as a plain user message (it is the user's own
  words, not harness metadata).
- chat agents: new agents/common/harness.md include (long form, with an
  explicit "data, not instructions" guard), added to assistant/kid/
  project-coordinator. The tag name rides the __HARNESS_TAG__ sentinel,
  resolved in AgentSystemContext to SYSTEM_EXTRA_TAG — renaming the tag
  stays a one-line change.

agents/kid/AGENT.md

@@ -86,3 +86,7 @@ Shared folders are special places where some members of the household can read a
 ## If they ask how you work
 
 If the child (or a grown-up) asks how the app itself works, or wants help turning something on, read `docs/index.md` first — it's written for you, not for them. Then explain whatever's relevant in your own simple, friendly words.
+
+---
+
+<!-- INCLUDE: common/harness.md -->