CL-6467: plugins list as grouped rows

[TheGreatAxios]

Summary

Closes the gap between the Plugins list and the new mock spec (§7.1). The list already rendered grouped, bordered rows per category (from CL-6272.1) rather than a card grid, so this is a targeted fix rather than a rewrite:

• Adds the page's own h1 ("Plugins") and verbatim .sub directory copy above the grouped rows.
• Removes the hidden … xl:block class hiding a row's status caption ("Connected · Connected here" / "Needs attention · Inherited" / "Not connected") below the xl breakpoint — per spec this is a core column, not overflow.

Detail page (§7.2, novel surface) is explicitly out of scope for this unit — a separate ticket/unit builds it, so Manage/Connect still open the existing connect panel rather than navigating to a route that doesn't exist yet.

What I did not verify

• Did not re-verify the plugin category taxonomy (Communication, Productivity, …) against the mock's example categories ("Project & issue tracking", "Messaging") — the mock only shows 4 illustrative plugins under 2 groups; recategorizing the real ~18-connector taxonomy felt like a separate product decision, not a layout change, so I left PLUGIN_CATEGORY_ORDER / CATEGORY_BY_ID untouched.
• Did not touch PluginLogo's tile styling (currently a bordered neutral square, not the mock's colored-brand-tile-with-initials pattern) — that's an icon/branding system change, not list-layout.
• Did not touch the "Connect apps" (MCP presets) or MCP servers sections that sit above the grouped list — they're real functionality the simplified mock doesn't depict; removing them looked out of scope for "convert the list to grouped rows."
• Ran only the scoped checks: bun test in packages/plugins-ui and apps/web/test/plugins-page.test.tsx, tsc --noEmit in packages/plugins-ui, and bun run build in apps/web — did not run the full-repo bun run check.
• No stack was booted; not manually clicked through in a browser.

Test plan

• bun test in packages/plugins-ui (12 pass, including 3 new tests: heading/sub copy, status caption never hidden, empty directory)
• bun test test/plugins-page.test.tsx in apps/web (5 pass, unchanged)
• tsc --noEmit in packages/plugins-ui (clean)
• bun run build in apps/web (succeeds)
• Full bun run check (not run — scoped only per instructions)
• Manual browser check on :3000 (owner is testing there; not done here)

Refs CL-6467 (does not close it).