feat(notifications): per-user notification preferences

Context: ROADMAP.md → Per-user notification preferences

Component

API or orchestration

Describe the feature

DynamoDB store (or equivalent) for per-user preferred channels, per-channel config (email address, Slack DM), and event filters beyond global defaults.

Use case

Teams have mixed preferences: some want every milestone, others only failures and approvals. Global fanout defaults are insufficient at scale.

Proposed solution

1. NotificationPreferencesTable: PK user_id, channel configs, subscribed event types.
2. FanOutConsumer resolves preferences after task submitter identity.
3. API: GET/PUT /users/me/notification-preferences; CLI bgagent notifications.
4. Merge with per-channel defaults from INTERACTIVE_AGENTS.md AD-4.
5. Document in docs/design/INPUT_GATEWAY.md.

Other information

• Depends on SES dispatcher for email channel.

• Design context: docs/design/INPUT_GATEWAY.md.

• This might be a breaking change

ROADMAP.md と docs/design/INPUT_GATEWAY.md から始め、次に FanOutConsumer と既存の通知および SES dispatcher のパスを追跡します。提案されているスコープには、NotificationPreferencesTable、GET/PUT API、CLI コマンド、イベントフィルターのマージ、ドキュメントが含まれます。完了とは、これらの要素が指定されたチャネルごとのデフォルト値で連携して動作することを意味します。