fix(planning): collapse archived cards behind a per-group count row in the interview Groups rail #8086

Refs: #7750

Agent Brief

Type: Bug
Summary: In the Groups rail, each group lists its live cards, then one collapsed "N archived" row.

Current behavior:
In the default Groups rail, archived question cards sit between live ones, the same size and only greyed: 52 archived among 35 live in the audited session.

Desired behavior:

• In Groups view, each group lists its live cards first. One row follows, reading "N archived" with that group's archived count.
• The row starts collapsed and expands on click to show the group's archived cards, and collapses again on a second click.
• A group with no archived cards shows no row.
• The Rounds view is unchanged. Filters still apply, so a filter that matches only archived cards shows the row expanded.

Key interfaces:

• The rail render in Groups view (archived question cards).
• The section render that groups cards, and the rail order the n keyboard shortcut and filters use.

Acceptance criteria:

• The diff adds a UI check at 1440x900 with a fixture holding archived questions in a group. It checks three things: the archived cards are hidden on load; one "N archived" row shows the right count; clicking the row shows the archived cards.
• The same check confirms that Rounds view still lists archived cards as before.
• scripts/check-changelog-fragments.sh passes, with one planning patch fragment added.
• CI's test-bash passes on the PR.

Out of scope:

• The archived banner text (sibling item).
• Disabling controls on superseded or archived questions (feat(planning): interview page does not enforce answer order or react to changed answers like the terminal interview #5796 G4) and count disagreements (feat(planning): interview page does not enforce answer order or react to changed answers like the terminal interview #5796 G11).

Refs #7750

This was generated by AI during backlog shaping. Split from #7750 so each piece fits the lane's run time; the owner approved this split in an attended backlog-shaping session on 2026-10-18.