Clarify field_option_names empty array behavior in tool description

The previous wording said 'Setting an empty array clears the field — use
delete: true for that instead', which was contradictory. An empty array
is actually rejected; clearing requires delete: true.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: owenniblock

pkg/github/__toolsnaps__/issue_write_ff_remote_mcp_issue_fields.snap

@@ -50,7 +50,7 @@
               "type": "string"
             },
             "field_option_names": {
-              "description": "Option names for multi-select fields. All names are validated against the field's options before the API call. Setting an empty array clears the field — use 'delete: true' for that instead. Cannot be combined with 'value', 'field_option_name', or 'delete'.",
+              "description": "Option names for multi-select fields. All names are validated against the field's options before the API call. An empty array is rejected — use 'delete: true' to clear the field. Cannot be combined with 'value', 'field_option_name', or 'delete'.",
               "items": {
                 "type": "string"
               },

pkg/github/issues.go

@@ -2051,8 +2051,8 @@ Options are:
 										Type: "string",
 									},
 									Description: "Option names for multi-select fields. All names are validated " +
-										"against the field's options before the API call. Setting an empty " +
-										"array clears the field — use 'delete: true' for that instead. " +
+										"against the field's options before the API call. An empty array " +
+										"is rejected — use 'delete: true' to clear the field. " +
 										"Cannot be combined with 'value', 'field_option_name', or 'delete'.",
 								},
 								"delete": {