feat: add opt-in gateway startup notifications

[vampyren]

Summary

• Add per-platform gateway_startup_notification opt-in setting
• Send the existing home-channel “Gateway online” notice after ordinary gateway startups, not only /restart
• Keep default false to avoid noisy lifecycle pings for public/end-user bots
• Add a 5-minute per-target debounce to avoid spam during crash loops

Why

Operators may have gateways restarted externally by systemd, host backup windows, VM maintenance, or process recovery. Today the gateway can notify after Hermes-initiated /restart, but external restarts have no .restart_notify.json marker, so operators may not know when the gateway is back.

Test Plan

• python -m pytest tests/gateway/test_config.py tests/gateway/test_restart_notification.py -q

[alt-glitch]

Related: #24271 (same opt-in gateway_startup_notification feature, global-level). Also overlaps with #27278 (unconditional approach). This PR adds per-platform granularity and 5-minute debounce on top of the same concept.

[vampyren]

Updated the commit author to use my GitHub noreply address so contributor attribution should pass once workflows are approved.

This PR intentionally overlaps with #24271/#27278, but takes the conservative operator-focused shape:
- opt-in only, default false
- per-platform configuration instead of a global-only toggle
- 5-minute per-target debounce to avoid crash-loop spam
- preserves the existing /restart notification behavior

Happy to adjust or close this if maintainers prefer one of the earlier approaches. Targeted validation:
python -m pytest tests/gateway/test_config.py tests/gateway/test_restart_notification.py -q

[teknium1]

Thanks for the focused opt-in design. The ordinary-start gap is still present on current main: gateway/run.py:7307-7313 sends the home-channel online message only when the planned-restart marker exists.

Problems

• The new tests call the private sender directly (tests/gateway/test_restart_notification.py:340 in this PR) rather than exercising the changed GatewayRunner.start() path. They do not establish that a no-marker ordinary startup reaches the sender after configuration is loaded.
• The new persistent setting has no documentation update. Existing lifecycle notification configuration is documented at website/docs/user-guide/messaging/index.md:575-590.
• Current main now has explicit planned-restart and chat-originated-restart marker handling at gateway/run.py:7292-7313; the salvage needs to preserve those distinct paths while adding the opt-in ordinary-start case.

Suggested changes

• Add real startup lifecycle coverage for default-quiet and opt-in ordinary starts, plus marker-precedence coverage.
• Document the finalized configuration surface alongside gateway_restart_notification.

Automated hermes-sweeper review.

[teknium1]

Please add a lifecycle-level regression that drives GatewayRunner.start() with no restart markers. Calling this helper directly does not verify that the ordinary-start branch invokes it after configuration is loaded.