CL-6368: shell chrome parity + team space isn't a workbench

[TheGreatAxios]

Summary

• Skills chrome parity: /skills list rows now use the same Table/TableRow idiom and LibrarySearchInput that /files and /agents already use, replacing the SidebarItemRow-based list and bespoke search box that made the page read differently from its siblings. /files and /agents were already on the shared StageTopBar + PageShell chrome and needed no change there; active nav-rail state was already correct on all three.
• Team space is not a workbench (CL-6368): the account's root tenant (parentId === null) is a container real workbenches live under as child tenants (CL-6089), never a workbench itself.
  • GET /workbenches (packages/insights) no longer includes the requested tenant's own rollup-of-itself row, so Insights' "Activity by workbench" chart only ever lists real child workbenches.
  • Onboarding now mints the account's root tenant under its own name ("Ada's team") instead of "Ada's workbench", which is what made it read like a workbench everywhere workbench names get listed.
  • Swept the sidebar/listings: WorkbenchList/listWorkbenches already queries workbench-kind child tenants directly rather than walking the tenant hierarchy, so it was never affected by this conflation.
  • No rename migration for existing dev/demo tenants — only fresh signups get the corrected name, per the repo's dev-data wipe convention.

Test plan

• apps/web: bun test — 581 pass
• packages/insights: DATABASE_URL=... bun test — 59 pass (includes the DB-gated /workbenches scope test updated to expect the team space excluded)
• bun run check:ui-vocabulary — ok ("team space" is a banned synonym per CL-6089; renamed the mint copy to avoid it)
• bun run check:react-ui-drift — ok
• tsc --noEmit clean for apps/web and packages/insights
• Scoped prettier --check clean on every changed file (repo-wide bun run lint has 6 pre-existing formatting warnings and 3 pre-existing eslint errors in files this PR never touches — confirmed via git diff --stat against those paths)