Defer busy CurrentSession reminders

[Aaronontheweb]

Summary

• Defer CurrentSession reminders when the target session cannot admit a distinct turn.
• Use Akka.Reminders NackAsync for durable backoff and attempt limits.
• Treat a missing supported gateway as a transient condition.
• Record one Netclaw failure only when scheduler retries end.
• Apply the same contract to Slack, Discord, Mattermost, SignalR, and TUI sessions.

Message contract

The session binding returns CommandDeferred before queue admission.

The execution actor maps this response to ReminderExecutionDeferred.

The reminder manager maps the internal message to NackAsync.

• RetryScheduled does not change Netclaw history or failure counts.
• Failed and Expired record one terminal failure.

Validation

• dotnet restore Netclaw.slnx
• dotnet build Netclaw.slnx --no-restore
• 214 affected actor tests passed.
• 1 SignalR session actor test passed.
• dotnet slopwatch analyze passed with one existing warning outside this change.
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate defer-busy-current-session-reminders

The evaluation suite did not run because the environment lacks the required provider variables.

Fixes #1980