fix: allow experimental flags to override umbrella

[rekram1-node]

Summary

• let explicit per-feature experimental flags override OPENCODE_EXPERIMENTAL
• keep OPENCODE_EXPERIMENTAL as the fallback when the per-feature flag is unset
• add regression coverage for explicit false overriding the umbrella flag

Tests

• bun test test/effect/runtime-flags.test.ts
• bun typecheck (packages/opencode)
• bun typecheck (packages/core)