[codex] Structure OAuth scope encoding failures - #3368

Reviewed the changed scope: packages/shared/src/oauthScope.ts and packages/shared/src/oauthScope.test.ts.

Findings:

• Imports: import * as Schema from "effect/Schema" correctly uses the subpath namespace import. The test's named imports from the local ./oauthScope.ts pure-helper module are appropriate (it is a pure helper/error module, not a Context.Service boundary), consistent with the convention that named imports are kept for pure helper/error/schema modules.
• Error modeling: OAuthScopeEncodingError is defined with Schema.TaggedErrorClass and structured attributes (scopes, invalidScopes, duplicateScopes) rather than storing an unstructured message as the only data. This is a pure validation/domain error created without an underlying failure, so omitting cause is correct per convention. The generic constant message getter is acceptable: the failures share the same semantics and the specific context lives in the structural fields. No redundant single-value operation/reason/kind/phase discriminator is introduced.
• Predicate: the test uses Schema.is(OAuthScopeEncodingError) directly rather than wrapping it in a redundant function, which is fine. (Optionally the module could export isOAuthScopeEncodingError, but a locally-scoped predicate in a test is not a convention violation.)
• Behavior change: replacing the thrown Error with the structured OAuthScopeEncodingError is covered by a focused added test, satisfying the behavior-change testing guidance; no service definition, layer, or file-migration conventions apply to this pure helper module.

No concrete convention violations were introduced or retained in the changed scope.

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Agent Credits: 28 credits