fix: hide skill prompt text from chat UI

Mark the skill template TextPart as synthetic so the full prompt is
sent to the model but hidden from the user. Add a short ignored
summary part ('Running skill: <command>') so the user still sees
confirmation that the skill was loaded.

Rebased from flexion/hide-skill-prompt-in-chat onto v1.4.0.

Author: lgarceau768

packages/opencode/src/session/prompt.ts

@@ -124,7 +124,7 @@ export namespace SessionPrompt {
 
       const resolvePromptParts = Effect.fn("SessionPrompt.resolvePromptParts")(function* (template: string) {
         const ctx = yield* InstanceState.context
-        const parts: PromptInput["parts"] = [{ type: "text", text: template }]
+        const parts: PromptInput["parts"] = [{ type: "text", text: template, synthetic: true }]
         const files = ConfigMarkdown.files(template)
         const seen = new Set<string>()
         yield* Effect.forEach(
@@ -1637,7 +1637,15 @@ NOTE: At any point in time through this workflow you should feel free to ask the
                 prompt: templateParts.find((y) => y.type === "text")?.text ?? "",
               },
             ]
-          : [...templateParts, ...(input.parts ?? [])]
+          : [
+              ...templateParts,
+              {
+                type: "text" as const,
+                text: `Running skill: ${input.command}`,
+                ignored: true,
+              },
+              ...(input.parts ?? []),
+            ]
 
         const userAgent = isSubtask ? (input.agent ?? (yield* agents.defaultAgent())) : agentName
         const userModel = isSubtask