Two complementary changes to reduce conversation cold-start friction:
A. Returning-user re-greeting (backend)
- When WS reconnects to a non-empty conversation and last_message_at is older
than chat_re_greeting_idle_hours (default 6h), the agent emits a warm
continuation message that references prior history instead of staying silent.
- Self-debouncing: the AI message updates last_message_at, so reconnects
within the window will not re-trigger.
- Skipped while profile collection is still pending.
D. Topic suggestion chips (backend + Expo)
- New WS message type topic_suggestions carries 3-4 quick-start chips derived
from the current memoir stage's empty slots (deterministic, no extra LLM
cost). Sent alongside opening / re-greeting / resume.
- Expo chat screen renders a horizontally-scrollable chip row above the input
bar; tapping a chip sends the chip's text as a user message and clears the
row. Sending any text/voice also clears the chips.
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