Remove the unused identity-token assess options

[vvillait88]

Summary

Removes the unused identity-token input from assess and aassess. The API no longer accepts it, so it is removed here:

• its two parameters on both methods;
• the request-body fields they set;
• the signature-material and provenance TypedDicts;
• the provenance block and identity_method value it produced on the response;
• the two agent-memory fields that advertised it.

Its tests go with it. Version 2.8.0: a minor, because nothing that works against the API today changes.

Worked with Varun, who asked for the feature to be removed everywhere.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

Removes the two assess parameters, two exported TypedDicts, the response provenance field, and two AgentMemoryHint fields. Nobody uses them, and the API ignores them.

Test plan

ruff check, ruff format --check, ty, vulture and pytest (172 passed, 2 skipped), all green locally.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed
• No secrets, credentials, or personal data in the diff or the tests