Prefer nearest .env in CLI eval hierarchy

[Copilot]

loadEnvFromHierarchy() was merging .env files across the eval/repo hierarchy, but effective precedence was backwards: parent values could block closer eval-specific values. This change makes the nearest .env take precedence while still inheriting missing keys from parent .env files.

• CLI env precedence

  • Adjust apps/cli/src/commands/eval/env.ts so hierarchical .env loading preserves this precedence:
    • exported shell environment
    • nearest eval-local .env
    • parent/root .env
  • Keeps hierarchical merging behavior intact for keys not defined locally.

• Regression coverage

  • Adds focused unit coverage for:
    • nearest .env overriding parent values
    • parent .env still supplying unset keys
    • existing process.env values remaining authoritative

• Behavior

  # repo root .env
  MY_VAR=root
  SHARED_ONLY=from_root

  # evals/foo/.env
  MY_VAR=local
  LOCAL_ONLY=from_subfolder

  Effective environment for an eval under evals/foo/ becomes:

  MY_VAR=local
  SHARED_ONLY=from_root
  LOCAL_ONLY=from_subfolder

---

📱 Kick off Copilot coding agent tasks wherever you are with GitHub Mobile, available on iOS and Android.

[cloudflare-workers-and-pages]

Deploying agentv with    Cloudflare Pages

Latest commit:	7cd0007
Status:	✅  Deploy successful!
Preview URL:	https://5655316e.agentv.pages.dev
Branch Preview URL:	https://copilot-ep-525-implement-fea.agentv.pages.dev

View logs

[christso]

@copilot since we don't override process.env would that mean we also don't override parent .env? When does parent env get loaded? Please do proper code review with manual e2e red green testing.