feat(core): support env interpolation across all config string fields

[christso]

Summary

Closes #506

• Adds centralized ${{ VAR }} interpolation across all YAML/JSONL config string fields
• New interpolateEnv() function recursively walks parsed objects and replaces ${{ VAR }} patterns
• Applied after YAML/JSONL parsing at every entry point: eval files, external case files, workspace configs, sidecar metadata
• Missing variables resolve to empty string (GitHub Actions convention); field validation catches blanks downstream
• Supports both full-value (${{ VAR }}) and partial/inline (prefix/${{ VAR }}/suffix) interpolation

Files changed

• packages/core/src/evaluation/interpolation.ts — new module with interpolateEnv()
• packages/core/src/evaluation/yaml-parser.ts — integration at 3 parse points
• packages/core/src/evaluation/loaders/case-file-loader.ts — integration for external YAML/JSONL
• packages/core/src/evaluation/loaders/jsonl-parser.ts — integration for top-level JSONL + sidecar
• packages/core/test/evaluation/interpolation.test.ts — 13 unit tests
• packages/core/test/evaluation/interpolation-integration.test.ts — 7 integration tests
• apps/web/src/content/docs/evaluation/eval-files.mdx — docs
• plugins/agentv-dev/skills/agentv-eval-builder/SKILL.md — AI skill reference

Test plan

• Unit tests for interpolateEnv (13 tests: replacement, partial, missing vars, recursion, immutability, edge cases)
• Integration tests through loadTests() (7 tests: criteria, workspace, external workspace, external YAML, external JSONL, passthrough, missing vars)
• Full test suite regression (1,167 tests pass, 0 failures)
• Build, typecheck, lint all pass

🤖 Generated with Claude Code

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