Add push channel for strategy plugin alerts

[Pigbibi]

Summary

• add a provider-neutral strategy plugin push alert channel
• support Pushover and ntfy without new package dependencies
• add channel-level dedupe/reporting and CRISIS_ALERT_CHANNELS selection

Tests

• PYTHONPATH=src pytest tests/test_strategy_plugin_email_notifications.py tests/test_strategy_plugin_sms_notifications.py tests/test_strategy_plugin_push_notifications.py tests/test_strategy_plugin_alert_dispatcher.py tests/test_strategy_plugins.py -q
• PYTHONPATH=src ruff check .

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Honor explicit empty channel selections

Treating ()/[]/"" as “unset” causes _resolve_channels to fall back to ("email", "sms", "push"), so callers that explicitly pass no channels (or load an intentionally empty crisis_alert_channels) now dispatch alerts instead of disabling delivery. This is a behavior regression from the previous logic where an empty selection normalized to no channels, and it can trigger unintended outbound notifications in production runs.

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