Summary

• add a core AuthWellKnown service with legacy auth.json migration into well-known.json
• add a shared core Substitution service for {env:...} and {file:...} replacements
• move well-known config loading out of opencode Auth and into AuthWellKnown.configs()

Verification

• bun typecheck (packages/core)
• bun typecheck (packages/opencode)
• bun test test/auth-well-known.test.ts test/auth.test.ts (packages/core)
• bun test test/config/config.test.ts test/plugin/trigger.test.ts test/plugin/workspace-adapter.test.ts test/agent/plugin-agent-regression.test.ts (packages/opencode)
• push hook: bun turbo typecheck