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.
Extract shared upload seam in skald-core, move Telegram and web
handlers to use it. Simplify media attachment routing. Clean up
unused deps and dead code.