Skill harness interface and repair retry (#53)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>

internal/ai/harness/echo_test.go

@@ -0,0 +1,51 @@
+package harness
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/ai/llm"
+)
+
+// echoSkill is the trivial test skill: it asks the model to repeat a word.
+type echoSkill struct{}
+
+type echoInput struct{ Word string }
+
+type echoOutput struct {
+	Echo string `json:"echo"`
+}
+
+var _ Skill[echoInput, echoOutput] = echoSkill{}
+
+func (echoSkill) Name() string { return "echo" }
+
+func (echoSkill) Version() int { return 1 }
+
+func (echoSkill) Render(in echoInput) (Prompt, error) {
+	if in.Word == "" {
+		return Prompt{}, errors.New("no word to echo")
+	}
+	return Prompt{System: "You repeat words.", User: "Repeat: " + in.Word}, nil
+}
+
+func (echoSkill) Schema() llm.JSONSchema {
+	return llm.JSONSchema{Name: "echo", Schema: json.RawMessage(
+		`{"type":"object","properties":{"echo":{"type":"string"}},"required":["echo"],"additionalProperties":false}`)}
+}
+
+func (echoSkill) Validate(out echoOutput) []string {
+	if out.Echo == "" {
+		return []string{"echo must not be empty"}
+	}
+	if out.Echo != strings.ToLower(out.Echo) {
+		return []string{"echo must be lower case"}
+	}
+	return nil
+}
+
+func (echoSkill) PostProcess(out echoOutput) (echoOutput, error) {
+	out.Echo = strings.TrimSpace(out.Echo)
+	return out, nil
+}