#25206 cmux NEXT Session Info show-more rows fill their hit target

Problem

Session Info's Show more / Show less controls only accepted clicks on their text and painted a hover row. Leo asked for the whole row as the click target with plain text: “the clickable area is actually the entire thing, but you don't have a thing on hover. So it's just kind of, like minimal feeling.”

Change

• Added shared `InspectorFold` for Changes, Subagents, and Background tasks.
• Made the shared fold control full-width, text-aligned, and chrome-free with no hover background.
• Kept native button keyboard behavior; keyboard focus follows the agent pane's shared policy (the soft pill, agent-session/shared/styles.css), with no focus rule of its own.

Validation

• CMUX_BROWSER_TESTS=1 bun test test/chat-inspector-fold-rows.test.ts (new, Chromium on the gallery's busy coordinator): each fold spans its list, a click in the empty end of the row opens and closes it, hover paints no fill, keyboard focus takes the shared pill. Its three row tests fail with base's inspector sources.
• `bun test test/chat-inspector-changes.test.tsx test/chat-inspector-tasks.test.tsx test/chat-inspector.test.tsx` (44 pass)
• `bun run typecheck`
• `bunx vp fmt --check`
• `bunx vp lint` (passes with four pre-existing warnings in unrelated files)
• `python3 scripts/cmux-next/check-text-focus-rings.py`
• Real gallery capture from `agent-pane.chat-inspector` at 2×; synthetic fixture data only. Before/after images will be attached under this PR.

Risk

Low: one shared inspector fold style/component; no state or placement changes.

Overlap audit: #25169 (test file), #24280 (Subagents), #25047/#25045 (Tasks), and #24100 (inspector CSS) touch adjacent files for unrelated behavior; none implement this fold interaction.

Agent session: huge-blue
Harness: Codex

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