Add autonomy toggle commands

[jamiepine]

Summary

• Add /autonomy for public status and authority-only /autonomy-on and /autonomy-off controls.
• Restore the last non-off level when autonomy is enabled, with observe as the first-use fallback.
• Serialize level changes with epoch and heartbeat admission so turning autonomy off stops new work while the current epoch settles.
• Keep configured and effective levels distinct when the instance ceiling applies.
• Register the controls across Discord, Telegram, and Slack, with text-command parity for other adapters.
• Align Discord thread admission, Telegram addressing, Slack argument validation, and Mattermost command replies.

Lifecycle

Autonomy transitions and supervisor admission share one lock. An off transition cannot race a new epoch or heartbeat claim. Existing turns and owned workers are not cancelled, but no additional wake events or heartbeat work enter the active epoch after the transition commits.

This PR is stacked on #650.

Testing

• cargo test agent::autonomy::tests
• cargo test api::config::tests
• cargo test commands::
• Targeted Discord, Slack, Telegram, Mattermost, and settings tests
• just preflight
• just gate-pr

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