feat(opencode): V2 commands.pre-execute alongside V1 trigger

packages/opencode/src/session/prompt.ts

@@ -1452,7 +1452,7 @@ const layer = Layer.effect(
         (part) => part.type !== "file" || !inputFiles.has(fileURLToPath(part.url)),
       )
       const isSubtask = (agent.mode === "subagent" && cmd.subtask !== false) || cmd.subtask === true
-      const parts = isSubtask
+      let parts = isSubtask
         ? [
             {
               type: "subtask" as const,
@@ -1477,6 +1477,15 @@ const layer = Layer.effect(
         { command: input.command, sessionID: input.sessionID, arguments: input.arguments },
         { parts },
       )
+      const decidedCommand = yield* events.waterfall(SessionEvent.Command.PreExecute, [
+        (_e, next) =>
+          Effect.gen(function* () {
+            const downstream = yield* next()
+            const downstreamParts = (downstream as { parts?: typeof parts } | undefined)?.parts
+            return { parts: downstreamParts ?? parts }
+          }),
+      ])
+      parts = (decidedCommand as { parts: typeof parts }).parts
 
       const result = yield* prompt({
         sessionID: input.sessionID,

packages/schema/src/session-event.ts

@@ -495,6 +495,29 @@ export namespace Permission {
   export type Requested = typeof Requested.Type
 }
 
+export namespace Command {
+  const CommandBase = {
+    ...Base,
+    command: Schema.String,
+  }
+
+  /**
+   * Live waterfall fired before a slash command runs. Listeners return
+   * `{ parts }` to rewrite the prompt payload, or throw to veto. This is
+   * the V2 replacement for the legacy V1 `command.execute.before` hook
+   * and gives plugins proper deny/transform semantics with `next()`.
+   */
+  export const PreExecute = Event.define({
+    type: "session.next.command.pre_execute",
+    schema: {
+      ...CommandBase,
+      arguments: Schema.String,
+      parts: Schema.Array(Schema.Unknown),
+    },
+  })
+  export type PreExecute = typeof PreExecute.Type
+}
+
 export const Retried = Event.define({
   type: "session.next.retried",
   ...options,