chore: remove dead mutationInputSource field from proposal entity data

The field was always set to null and never read after the planner LLM
was removed. Dropping it keeps the persisted proposal shape clean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MaxLinCode

apps/web/src/lib/server/conversation-state.ts

@@ -101,7 +101,6 @@ export function deriveConversationReplyState(
           replyText: input.reply,
           policyAction: input.policy.action,
           targetEntityId: input.interpretation.resolvedEntityIds[0] ?? null,
-          mutationInputSource: null,
           confirmationRequired: true,
           originatingTurnText: input.userTurnText,
           missingFields: input.policy.clarificationSlots,