fix(create-slack-app): drop --experiment=sandboxes flag (#87)

* fix(create-slack-app): drop --experiment=sandboxes flag

The `sandboxes` experiment was removed from `slackapi/slack-cli` — the flag
is no longer registered (`internal/experiment/experiment.go`) and none of
the `cmd/sandbox/*` commands gate on it. Passing `--experiment=sandboxes`
is dead weight, and copy/pasting it into a modern CLI just adds noise.

Uses plain `slack sandbox list` and `slack sandbox create` in the skill.

* chore: add changeset for sandbox experiment flag removal

Author: mwbrooks

.changeset/drop-sandbox-experiment-flag.md

@@ -0,0 +1,5 @@
+---
+"slack": patch
+---
+
+Drop the `--experiment=sandboxes` flag from `slack sandbox` invocations in the `create-slack-app` skill. The experiment has been removed from `slack-cli`, so the flag now surfaces an unknown-experiment warning that can confuse users and agents.

skills/create-slack-app/SKILL.md

@@ -41,13 +41,13 @@ Wait for confirmation that authentication succeeded before proceeding.
 
 ## Step 3: Set Up a Developer Sandbox
 
-Run `SLACK_CMD sandbox list --experiment=sandboxes` to check if the developer already has a sandbox.
+Run `SLACK_CMD sandbox list` to check if the developer already has a sandbox.
 
 - **If a sandbox exists**: Show it and confirm they want to use it.
 - **If no sandbox exists**: Tell the developer to create one:
 
   ```text
-  ! SLACK_CMD sandbox create --experiment=sandboxes
+  ! SLACK_CMD sandbox create
   ```
 
   Alternatively, they can create one at <https://api.slack.com/developer-program/sandboxes> or join the Developer Program at <https://api.slack.com/developer-program/join>.