* refactor(config): retire flat streaming keys from the last six channel schemas
signal, irc, nextcloud-talk, whatsapp, googlechat, and mattermost now accept
only the nested streaming.{chunkMode,block.enabled,block.coalesce} shape
(mattermost also drops scalar/boolean streaming); flat spellings migrate via
each channel's defineChannelAliasMigration doctor contract with root seeding
for their wholesale-replace account merges.
* feat(channels): warn once per key when the deprecated flat streaming fallback is used
Bundled schemas now reject the flat delivery keys, so the streaming.ts
fallback only serves external SDK plugin configs; emit a once-per-process
per-key subsystem warning and pin the removal plan to the next release train.
* chore(config): regenerate bundled channel config metadata for nested-only streaming
* docs: describe nested-only channel streaming config and the SDK flat-key deprecation window
* fix(whatsapp): seed migrated named-account streaming from the accounts.default layer
WhatsApp resolution layers accounts.default shared config between root and
named accounts, so doctor-materialized account streaming objects now inherit
default-account settings over root ones; mattermost schema test moves to the
nested-only shape with explicit rejection coverage; scope flat-key deprecation
notes to the pending Matrix/Feishu migrations.
* fix(whatsapp): resolve the default account case-insensitively when seeding migrated streaming
* chore(plugin-sdk): repin API baseline for nested-only channel streaming types
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