Fix #57: default-expand My Plugins on first encounter

[lucasmendes-design]

Summary

• Flip the first-encounter default for the My Plugins group from collapsed to expanded, so users immediately see what the grouping contains.
• Other groups still default to collapsed.
• All other state paths are unchanged: a user-driven collapse still persists per-tab via sessionStorage, and URL-based auto-expand still wins over both stored state and the new default.

Closes #57.

Implementation

Single-line behaviour change in decideInitialState: when no stored state and no URL auto-expand, return true for groupId === 'plugins', otherwise false. The existing plugins-only special case in grouping.js (customize button) made hardcoding the id here consistent with the existing style.

Header doc comment and __tests__/expand-collapse.test.js updated to match the new contract.

Test plan

• Activate the plugin on a fresh tab, clear sessionStorage, reload /wp-admin/ → My Plugins group renders expanded on first paint.
• Click the My Plugins toggle to collapse it, reload → group stays collapsed (stored state honored).
• Open /wp-admin/ in a fresh tab → group default-expands again (sessionStorage is per-tab).
• Navigate to a page belonging to a non-plugins group (if any are configured) → that group does not auto-expand; My Plugins still default-expands.
• Navigate to a page whose slug matches a child of My Plugins → group expands as before (auto-expand path unchanged).

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