Revert the over-redaction from 3c22d88: a customer-initiated
session (social / lead who messages in) now greets with the
persona's generated opener — a realistic 'สวัสดี สนใจ ขอถามราคา'
line — instead of the static 'ลูกค้าเริ่มต้นบทสนทนาแล้ว'
placeholder. The opener is spoken public dialogue by design
(OPENER RULE keeps pains/budget/personality out of it), so
remove the redact_initial_customer strip from serialize_session.
Phone/f2f (seller-initiated) is unchanged: no customer line,
the trainee opens the call. Untrusted system-note filtering and
internal-state/provider-path stripping are untouched.
Tests: 5 redaction regressions flipped to the new contract.
517 passed (3 pre-existing demo-date failures, unrelated).
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