fix(linear): restore OAuth auth in idempotent create path

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Summary

• fix createIssueSingleAttempt to use authHeader() instead of directly using APIKey
• this restores OAuth client-credentials authentication for idempotent create flows (CreateIssueIdempotent)
• add regression test TestCreateIssueIdempotentOAuthUsesBearerOnCreate

Bug / impact

A recent Linear idempotent-create helper bypassed the shared request executor and set Authorization from c.APIKey directly. In OAuth mode, APIKey is empty, so idempotent creates returned 401 and failed to push new issues.

Validation

• CGO_ENABLED=1 go test -tags gms_pure_go ./internal/linear
• make test (fails in existing unrelated suites due to environment/dependencies: cmd/bd unreachable-host expectation and internal/storage/dolt missing dolt binary)