docs(website): the email channel and the Teams migration as built (engagement Phase 6)
ENGAGEMENT.md gains the Phase 6 as-built: the seven decisions settled up front, the audience problem that shaped the phase, why scope_key is not subject_key, the one place decision 4 as phrased could not ship, the projection's rule and what it refuses to guess, the digest correcting Phase 4a rather than only implementing §4.2b, and the three defects the build found. §4.2b and §6.0b updated with it. TEAMS.md §6.4 rewritten: the pipeline it described no longer exists as its own thing. It now says which of the three sinks moved and which did not, where each of the four properties it always claimed lives now, that Team email is OFF until an operator turns a rule on, and what the generalized unsubscribe token does. §6.3's `last_digest_at` is marked as no longer read. BACKEND_DESIGN.md: engagement_digest_state, engagement_outbox.scope_key, and the unsubscribe routes — the canonical /public/engagement pair plus the /public/teams path kept permanently because mail is not editable once sent. Code: RunicGateway/website#TBD Co-Authored-By: Claude <noreply@anthropic.com>
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