api: align the public API with Grain terms

[suraciii]

Summary

• rename the public identity model to GrainId with GrainType and GrainKey
• use GrainId across runtime, store, generated code, forwarding, and tests
• change the generator marker from //gor:entity to //gor:grain
• update generator help, comments, fixtures, and generated bindings
• leave Reminder, State, Request Context, Call Filters, and reentrant Calls unchanged

Verification

• diff check passed
• no Identity, Identities, EntityType, or gor:entity symbols remain in Go files
• make ci passed

Closes #91