FL-1503: replies without threads — replyToCommentId replaces parentId

Replies to comments (FL-1425) reworked as in Telegram (Product, 2026-10-19): a reply is a comment of the post's one stream that quotes the comment it answers — there are no threads.

cms.PostCommentCreated

• replyToCommentId (the answered comment) replaces parentId (a thread's top-level comment); IsReply() reads it.
• ErrReplyFieldsWithoutParent → ErrReplyFieldsWithoutTarget ("reply fields require replyToCommentId").
• replyToCreatorId, replyToPostedAsCommunity, postedAsCommunity, community: {name, logo} unchanged.

parentId (v1.68.0) is read only by the unmerged FL-1503 / FL-1504 branches, so removing it breaks no consumer.

Tag for the stand: v1.69.0-rc.1 (this head). After merge → v1.69.0; CMS and NMS bump to it.

Contract: https://freedomgroupinternational.atlassian.net/wiki/spaces/fig/pages/374112259

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