PSM anchorBrand term not derived from Zoe offer-safe issuers

[dckc]

Is this a bug? Perhaps more of a best practices violation?

Describe the bug

The PSM contract uses an anchorBrand rather than using the brands Zoe provides based on the issuers given to startInstance(). Zoe provides offer safety only for registered issuers (either via startInstance or saveIssuer).

To Reproduce

This is hypothesized, not observed

1. Instantiate the contract with anchorBrand: b1 but don't provide any corresponding issuers in startInstance()
2. Client make an offer based on anchorBrand from terms.
3. offer fails due to unregistered brand

Expected behavior

Clients should only make offers using getTerms().brands

Platform Environment

• what OS are you using? what version of Node.js?
• is there anything special/unusual about your platform?
• what version of the Agoric-SDK are you using? (run git describe --tags --always)

agoric-sdk/packages/inter-protocol/src/psm/psm.js

Line 124 in c5964fe

const { anchorBrand, anchorPerMinted } = zcf.getTerms();

Additional context

Discovered while reviewing KREAd
#8195 (comment)

cc @turadg