Save private review reply drafts on existing MR discussions

[sjungwon03]

Problem

Pending review supports general and inline draft notes plus publication, but existing MR discussions only offer immediate public replies.

Acceptance criteria

• Save exact private reply Markdown through the documented draft-note POST with in_reply_to_discussion_id, explicit non-resolution and strict created-target confirmation; no public fallback, authentication replay, redirect or automatic retry.
• Keep captured account/client, route IID and global MR identity separate. Confirm a fresh complete selected discussion before dispatch; refuse stale, missing, individual or system-note targets.
• Reuse the shared reservation and actual settlement barrier. Uncertain private reply saves require complete private-note and original public-target inspection plus fresh consent before manual retry. Preserve publication/reviewer inspection gates and do not select another target if the original disappears.
• Add localized private reply controls to the existing shared discussion/composer UI; preserve unsent public input and discard obsolete session/origin input and results.
• Work test-first, update documentation, check all five locales/three widths/both themes/keyboard/large text, inspect synthetic captures, run all quality gates and exact-head CI, review/approve with sjungwon03-ai and merge with sjungwon03.

Private commit/image/file creation and resolution during draft creation remain separate slices. No live-instance or durable exactly-once guarantee is implied.