feat: allow env placeholders in config files

[Helweg]

Summary

Allow .opencode/codebase-index.json to reference environment variables with exact {env:VAR_NAME} placeholders so secrets such as custom provider API keys do not need to be committed.

Changes

• add config placeholder resolution for exact {env:VAR_NAME} string values during parseConfig
• resolve only the config fields that are actually used so inactive branches like unused customProvider settings do not fail startup
• reject malformed placeholder-like strings early, add parser coverage, and document the supported syntax in the README

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #40