docs(design): describe the Door as the two-button group it is

[dormouse-bot]

DESIGN.md §5 Doors still describes the Door as it was before the notepad and browser-Surface work landed, so the one file AGENTS.md tells you to read before touching any text-*/bg-* class on a Door is wrong about that component's structure. This updates the three stale bullets to match lib/src/components/Door.tsx.

Three claims that no longer hold:

• "The whole door is a button." Since the notepad work, the door is a labelled role="group" wrapper holding one or two buttons — the title button reattaches and starts the drag, the notepad button does neither. docs/specs/notepad.md → Notepad UI already states this correctly ("A Door is a wrapper carrying data-door-id with one or two buttons"), so DESIGN.md was the side that drifted; the new text points at that spec rather than restating its rule.
• "horizontal padding px-2.5 (10px)". The padding moved onto the inner buttons and now varies: the title button is pl-2.5 with pr-2.5 alone or pr-1 when the notepad button follows, and the notepad button carries pl-0.5 pr-2.
• Content list. It omitted the leading browser-display icon cluster and the trailing notepad button, both of which a Door renders today.

Also notes that the notepad button is the only part of a Door taking the standard hover:bg-current/10 wash — the "no decorative hover" rule still holds for the door itself.

No test: this is a documentation-only change, and DESIGN.md carries no lint beyond the generic ones. Full pnpm test is green on the branch head.

Evidence

Door.tsx at 61dbd293 — outer element is role="group" with data-door-id; the title <button> has pl-2.5 plus showNotepad ? 'pr-1' : 'pr-2.5'; the notepad <button> has pl-0.5 pr-2 hover:bg-current/10; BrowserDisplayIcon renders ahead of the title.

BrowserDisplayIcon in lib/src/components/wall/BrowserDisplayIcon.tsx is a two-glyph cluster, not one glyph: an inline-flex gap-0.5 span holding AgentRobotIcon plus the mode's presentation glyph, with the robot dropped for iframe alone. The Content bullet says so.

Timeline: the Door's notepad button landed 2026-09-04 (2d0945a3, 61dbd293) and the browser-display cluster 2026-09-03 (c7fa07d4); DESIGN.md's last edit is 2026-09-05 (c4d0b2b1) and touched other sections only, so the Doors entry was never carried forward.

The components.door front-matter token (padding: "0 10px") is left alone — it is the nominal design token for the component, not a per-button inventory, and changing it looked like overreach for a doc-accuracy fix.

The second commit on this branch folds in the two findings this PR's own draft review raised: "glyph" singular understated the icon cluster, and the first draft of the Dimensions bullet wrongly claimed the notepad button leaves the door's width unchanged (it adds roughly 22px unless max-w-[220px] truncates the title instead), so that clause is gone.

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[dormouse-bot]

Reviewing as a draft — flagging anything that looks worth a quick fix. Mark ready for a full review.

Both findings are on the two rewritten bullets; the role="group" / hover rewrite and the notepad-button description check out against Door.tsx, as do the two spec headings the new text points at.