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.

src/frontend/api/ws.rs

@@ -380,7 +380,7 @@ async fn handle_socket(mut socket: WebSocket, skald: Arc<Skald>, source: String,
                 // ── Regular LLM message ───────────────────────────────────────
                 // Attachments uploaded beforehand, plus an optional custom-command
                 // marker. Persisted on the user turn as MessageMetadata; the
-                // [SYSTEM INFO] block the LLM sees is generated on the fly by the
+                // `<system-extra>` block the LLM sees is generated on the fly by the
                 // projection (never stored as text), and the UI renders the
                 // command's `display` instead of the expanded `content`.
                 let attachments = client_msg.attachments.clone();