Remove the unused identity-token minting

[vvillait88]

Summary

Removes the unused identity-token code from the CLI:

• the minting, request-signing and presenting modules;
• the agent key path, and the request-descriptor helper only they used;
• their tests;
• the knip exclusion that kept them as a dormant surface.

No command exposed them. Takes @agent-score/sdk ^2.9.0. Version 0.7.0: a minor.

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

No CLI command, flag or MCP tool changes.

Test plan

lint, typecheck, knip (now with nothing excluded), test (547 passed, 2 skipped) and build, 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