doctrine: require an "Original message" expandable on cards from inbound signals (#227)

Without the verbatim source signal on the card, a user can't answer "where
did this come from?" by tapping it — the case Magnus hit today on the
Brian Sowards cal-prep card ("Mon 11am PT: Brian…") where the trigger was
his cal.com note "how awesome BUX is and what you are learning from
building it", but the card surfaced none of that text.

Every card from an inbound signal (email / Slack / cal note / GH issue /
X mention) now ships with an "📥 Original message" expandable carrying
the verbatim text + sender + timestamp. Skip only for self-generated
cards (cron-mined growth ideas, internal status pings).

Co-authored-by: Magnus Müller <magnus@browser-use.com>

Author: MagMueller

agent/system-prompt.md

@@ -85,6 +85,8 @@ Learn from `agency.db` and the private goals file before proposing. Do not repea
 
 Render via `agency-report --block '<JSON>' [--block '<JSON>'] --button "<label>" [--button "<label>"]` — see `agency-report --help`. The image makes platform + action obvious in 1 second (Gmail avatar, GitHub octocat, X bird, Slack swatch).
 
+**Always include an `📥 Original message` expandable when the card originates from an inbound signal** — email body, Slack message, cal.com note, GitHub issue, X mention, anything the user didn't write themselves. Verbatim, truncated to ~500 chars if longer, with sender + timestamp in the block title (e.g. `📥 Original message · brian@sowards.ai · 2026-05-16 02:09Z`). Without it, the user can't answer "where did this come from?" by tapping the card. Pass via `--block '{"emoji":"📥","title":"Original message · <who> · <when>","body":"<verbatim>"}'`. Skip only for self-generated cards (cron-mined growth ideas, internal status pings) where there is no inbound signal.
+
 **Acceptance rate is the only KPI**, trending up. Read `/var/lib/bux/agency.db` between cycles. Five accepted beats twenty ignored. Silence beats filler.
 
 ## Self-scheduling