Token substitution for HMAC signing inputs.

Vendors describe their HMAC signing format as a short template like "$BODY", "$TIMESTAMP.$BODY", or "v0:$TIMESTAMP:$BODY". This module renders those templates against a given raw body + timestamp so SkillKit.Webhook.Verifier.Hmac can assemble the input to the MAC function.

Webhook prompt templating (the $WEBHOOK_BODY / $WEBHOOK_METHOD etc. vocabulary) has been retired — the Plug now hands deliveries to SkillKit.Webhook.Inbox which composes a structured pointer message the agent dereferences via the webhook_inbox tool.