Add strategy plugin email alert publisher

[Pigbibi]

Summary

• add a shared strategy plugin email alert publisher with structured sent/skipped/failed results
• add alert context labels and stable alert keys for idempotency markers
• document the shared SMTP escalation path

Tests

• PYTHONPATH=src python -m pytest tests/test_email.py tests/test_strategy_plugins.py -q

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Enable local marker checks when no backend is configured

When StrategyPluginEmailAlertMarkerStore is instantiated with defaults (local_dir=None, gcs_prefix_uri=None), this branch makes has_alert() always return False, so duplicate suppression never activates for repeated runs. Because _local_path() already falls back to tempfile.gettempdir(), callers can reasonably expect default local persistence; instead, alerts are re-sent every time unless local_dir is explicitly set, which undermines the new idempotency behavior.

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