docs: add documentation for feature/select-primitive

Author: Jaakko Heusala

docs/select-syntax.md

@@ -0,0 +1,38 @@
+The `select` operation chooses between two values based on a simple string 
+condition. It enables basic control flow within Gendo pipelines by selecting 
+either a `trueValue` or a `falseValue` depending on whether the `condition` 
+string exactly matches `"true"` (case-insensitive). This operation is 
+compatible with local text-based models, which typically produce plain textual 
+answers.
+
+The syntax of the `select` operation is as follows:
+
+  select [destination] condition trueValue falseValue
+
+The `destination` identifier is optional. If omitted, the result is bound to 
+the special slot `_`. The `condition` is required and must be a 
+string—typically the result of a previous model call or logic step. If 
+`condition` equals `"true"` (case-insensitive), the `trueValue` is chosen. 
+Otherwise, the `falseValue` is chosen. The selected value is then stored in 
+`destination` or in `_` if no destination is provided.
+
+An example with an explicit destination:
+
+  select outcome "true" "Proceed" "Abort"
+
+In this example, the string `"Proceed"` is assigned to `outcome` because the 
+condition matches `"true"`.
+
+Another example using a prior model result as the condition:
+
+  prompt isValid "Is the previous input acceptable? Reply 'true' or 'false'."
+  select validationMessage isValid "Input is acceptable." "Input is not acceptable."
+
+Here, the pipeline sends a prompt to the model. The response, assumed to be 
+`"true"` or `"false"`, is stored in `isValid`. The `select` instruction then 
+chooses an appropriate message and binds it to `validationMessage`.
+
+The `select` operation requires all arguments to be present and in order. It 
+performs no transformation beyond the selection logic, and it produces exactly 
+one value. All bindings follow the single-assignment rule: once a name is used 
+as a destination, it cannot be reassigned later in the same pipeline.