chore: rename IAM to instance (#11351)

# Which Problems Are Solved

In many cases IAM was used where instance was meant. 

# How the Problems Are Solved

- Rename wrong usage of IAM to instance all over the code base.
- Add an agents.md which explains the terms used in Zitadel's
architecture.

# Additional Context

- closes #11280

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: muhlemmer <[email protected]>
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Marco A. <[email protected]>

internal/api/grpc/instance/v2/integration_test/query_test.go

@@ -105,7 +105,7 @@ func TestGetInstance(t *testing.T) {
 				inputContext:      ctxWithSysAuthZ,
 				inputInstanceID:   "invalid",
 				expectedErrorCode: codes.NotFound,
-				expectedErrorMsg:  "Errors.IAM.NotFound (QUERY-n0wng)",
+				expectedErrorMsg:  "Errors.Instance.NotFound (QUERY-n0wng)",
 			},
 		}