Undo opt-out

Undo an opt-out the reply reading recorded on a thread (a misread, such as "remove me as the contact, write to partners@"). Only opt-outs this account's own reply reading listed are removed; a click on the unsubscribe link or another sender's opt-out stays. The thread goes back to replied with no more sequence steps, and the contact can be emailed again. Guarded: without approval_token it returns {error:"approval_required", approval_token, summary} for the user to confirm; call again with the token. Every dashboard guardrail still applies: this is exactly what a user clicking the dashboard could do, no more. GUARDED: requires the account's Unrestricted MCP setting; returns 403 {error:'restricted'} otherwise.