* feat(channels): add dmPolicy config to disable private/DM messages
Add DmGate class mirroring GroupGate to gate DM/private messages in
channel adapters. Operators can now set dmPolicy: 'disabled' in their
channel config to silently drop all DM messages while keeping group
messages active.
Closes#6392
* fix(channels): address review feedback for dmPolicy
- Add dmPolicy: 'open' to all test config factories (8 files) to
maintain type correctness with required ChannelConfig field
- Add integration tests in ChannelBase.test.ts:
- preflightInbound: DM dropped + group passes when dmPolicy=disabled
- isStoredLoopTargetAuthorized: DM loop job disabled + group passes
- Add dmPolicy assertions in config-utils.test.ts (default + explicit)
- Keep dmPolicy as required field (not optional) for strict parity
with groupPolicy
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