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.

crates/skald-core/src/loop_adapters/snapshots/inlined_attachment.json

@@ -4,7 +4,7 @@
     "role": "system"
   },
   {
-    "content": "old shot\n\n[SYSTEM INFO]\n1 attached file:\n* uploads/1/shot.png",
+    "content": "old shot\n\n<system-extra>\n1 attached file:\n* uploads/1/shot.png\n</system-extra>",
     "role": "user"
   },
   {

crates/skald-core/src/loop_adapters/system.rs

@@ -110,6 +110,8 @@ impl SystemContextSource for AgentSystemContext {
             }
         }
 
+        static_content = resolve_harness_tag(static_content);
+
         // The scratchpad sits before the conversation: shared by every agent of
         // the session, and re-read every turn (it changes, so it is its own
         // message rather than part of the cached prefix).
@@ -386,10 +388,44 @@ fn non_empty(s: &Option<String>) -> Option<&str> {
     s.as_deref().map(str::trim).filter(|s| !s.is_empty())
 }
 
+/// Replaces the `__HARNESS_TAG__` sentinel with the canonical harness-data tag
+/// name (`SYSTEM_EXTRA_TAG`). A no-op when the prompt never mentions the
+/// sentinel, so it is safe to run unconditionally on every system context.
+///
+/// `common/harness.md` (included by the chat agents) documents the tag through
+/// this sentinel, so the instruction the model sees and the tag actually
+/// emitted by `system_extra()` can never diverge: both read `SYSTEM_EXTRA_TAG`.
+fn resolve_harness_tag(content: String) -> String {
+    if content.contains("__HARNESS_TAG__") {
+        content.replace("__HARNESS_TAG__", core_api::message_meta::SYSTEM_EXTRA_TAG)
+    } else {
+        content
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn harness_tag_resolves_to_canonical_tag() {
+        // Every occurrence of the sentinel is replaced with the tag emitted by
+        // `system_extra()` — single source of truth: `SYSTEM_EXTRA_TAG`.
+        let input = "Data lives in <__HARNESS_TAG__>…</__HARNESS_TAG__> blocks.";
+        let out = resolve_harness_tag(input.into());
+        let tag = core_api::message_meta::SYSTEM_EXTRA_TAG;
+        assert!(out.contains(&format!("<{tag}>")), "{out}");
+        assert!(out.contains(&format!("</{tag}>")), "{out}");
+        assert!(!out.contains("__HARNESS_TAG__"), "sentinel survived: {out}");
+    }
+
+    #[test]
+    fn harness_tag_is_noop_when_absent() {
+        let input = "Plain prompt, no sentinel here.";
+        let out = resolve_harness_tag(input.into());
+        assert_eq!(out, input);
+    }
+
     #[test]
     fn shared_folders_table_renders_access_and_description() {
         use crate::db::shared_folders::SharedFolderAccess;