fix(planning): add jump-to-newest and collapse older notes in the interview Notes panel #8084

Refs: #7756
Depends on: #8083

Agent Brief

Type: Bug
Summary: The Notes panel offers a jump to the newest note and collapses older notes.

Current behavior:
The Notes panel renders every note expanded in one column: 7,625 px tall in the audited session. There is no way to reach the newest note except by scrolling.

Desired behavior:

• The Notes panel shows a control that scrolls the newest note into view. It works from the keyboard.
• Older notes collapse to one line each and expand when clicked. The newest note always renders expanded.
• With only a few notes, the panel needs no collapsing. The agent picks the threshold and states it in the PR.
• Pinned and unpinned panels both behave this way.

Key interfaces:

• The Notes panel render: the note list, and each note's collapsed and expanded state.

Acceptance criteria:

• The diff adds a UI check with a fixture holding enough notes to collapse. It checks three things: the jump control brings the newest note into the panel's viewport; an older note renders collapsed; clicking that note expands it.
• scripts/check-changelog-fragments.sh passes, with one planning patch fragment added.
• CI's test-bash passes on the PR.

Out of scope:

• Flyout overlap at 1440 px (the item this depends on, which touches the same panel render).
• Note content, the note reply flow, and the Activity panel.
• Layout at 390 and 425 px (sibling item).

Refs #7756

This was generated by AI during backlog shaping. Split from #7756 so each piece fits the lane's run time; the owner approved this split in an attended backlog-shaping session on 2026-10-18.