Guard Plugins page reloads against a stale tenant's late fetch

[TheGreatAxios]

Fixes CL-7138 — https://linear.app/abklabs/issue/CL-7138

Problem

apps/web/src/pages/plugins-page.tsx:95-121 ran reloadPlugins/reloadSkills as .then(setState).catch(setState) chains with no cancellation. Both effects re-ran on selectedTenantId, but a fast bench switch could leave the previous tenant's fetch in flight; if it resolved after the new tenant's fetch, its response overwrote the newly selected tenant's state, so the Plugins page could momentarily (or persistently, on a slow response) show the wrong bench's plugins/skills.

Change

• Both reload effects now track a cancelled flag set in their cleanup (the pattern already used in packages/settings-ui/src/people-section.tsx:106-154) and only call setState when !cancelled.
• Restructured so each effect owns its own fetch, keyed on selectedTenantId plus a pluginsReloadKey/skillsReloadKey counter; the existing imperative reloadPlugins/reloadSkills call sites (connect/disconnect panel, retry button, skill creation) now just bump that counter, so they still refresh the current tenant without duplicating fetch logic outside the effect.
• Each effect also tracks the tenant it last fetched for in a ref, and only sets status: "loading" when selectedTenantId actually changed — a same-tenant reload (connect/disconnect, Retry) keeps whatever is on screen and swaps in the fresh result when it lands, instead of tearing the page down to the full loading skeleton on every reload.
• No copy changes.

Tests

apps/web/test/plugins-page.test.tsx:

• Mounts the page on tenant A (skills fetch deferred), switches to tenant B before A resolves, resolves B's fetch first and A's late — asserts the page ends up showing B's skill and never A's. Confirmed this fails against the pre-fix code and passes against the fix.
• Drives the real GitHub disconnect flow (Manage → Disconnect → confirm), deferring the reload's fetch, and asserts the gallery still shows its prior content (not the "Loading plugins…" skeleton) while the reload is in flight, then shows the disconnected result once it lands. Confirmed this fails without the loading-skeleton guard and passes with it.

cd apps/web && bun test ./test/plugins-page.test.tsx — 9 pass.
cd apps/web && bun run typecheck — clean.
bunx prettier --write + bunx eslint on both changed files — clean.

Full bun run check not run locally (shared-machine load).