Add PPR evals and fix env loading for agent-eval 0.9.5 (#92063)
Upgrades `@vercel/agent-eval` from 0.8.0 to 0.9.5. The new version loads
`.env` / `.env.local` from its own cwd (`evals/`) rather than inheriting
from the parent process, so the old `process.loadEnvFile()` call no
longer works. Instead, `run-evals.js` now symlinks the root env files
into `evals/` before spawning the harness. `evals/.env` is added to
`.gitignore` to keep secrets out of the tree.
Two new evals exercise PPR knowledge: `agent-041-optimize-ppr-shell`
checks that the agent decomposes a monolithic `loading.tsx` into
granular `Suspense` boundaries, and `agent-042-enable-ppr` verifies the
agent knows PPR is enabled via `cacheComponents: true` (not the old
`experimental.ppr` flag).
