docs: clarify env var fallback for eval interpolation

[christso]

Summary

• clarify in docs that ${{ VAR_NAME }} resolution reads exported process environment variables directly
• note that .env files are optional and only augment the environment when present
• add an explicit regression test covering interpolation with no .env file

Closes #927.

Red

Issue #927 described .env as required in CI. That matched the docs wording before this PR:

• targets/configuration.mdx said ${{ VARIABLE_NAME }} reads values "from your .env file"
• quickstart.mdx instructed users to define target env names in .env

Green

Verified on current origin/main and this branch that no .env file is required for interpolation:

CI_TEMPLATE_PATH=/tmp/template bun -e "import { loadTestSuite } from './packages/core/src/evaluation/yaml-parser.ts'; const suite = await loadTestSuite(process.argv[1], process.cwd()); console.log(suite.tests[0].workspace?.template);" /tmp/env-path.eval.yaml

This resolved workspace.template to the exported path directly.

Verification

• bun test packages/core/test/evaluation/interpolation-integration.test.ts
• Manual repro: loadTestSuite() resolved ${{ CI_TEMPLATE_PATH }} with no .env present

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