sbx: clarify sandbox auto-naming in docs (#25074)

## Description

Running `sbx run claude` from `~/my-pro` produces a sandbox named
`claude-my-pro`, but the docs tell you to clean up with `sbx rm
my-sandbox` and never explain where that name came from.

This PR documents the `<agent>-<directory>` and updates examples across
the sandbox docs to use names that match the rule, so the flow lines up
with what users actually see in `sbx ls`.

<img width="1391" height="703" alt="Screenshot 2026-05-16 at 2 48 30 PM"
src="https://github.com/user-attachments/assets/b0357bb5-32ea-48f4-ab5d-8494431e2772"
/>


## Reviews

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- [x] Technical review
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Author: Pradumnasaraf

content/manuals/ai/sandboxes/get-started.md

@@ -199,7 +199,7 @@ directory without touching your main working tree.
 When the session ends, review what the agent did from the worktree:
 
 ```console
-$ cd .sbx/<sandbox-name>-worktrees/my-feature
+$ cd .sbx/claude-my-project-worktrees/my-feature
 $ git log
 $ git diff main
 ```
@@ -243,15 +243,19 @@ set and how to customize it.
 Sandboxes persist after the agent exits. To stop a sandbox without deleting it:
 
 ```console
-$ sbx stop my-sandbox
+$ sbx stop claude-my-project
 ```
 
+The sandbox name comes from the agent and workspace directory — see
+[Reconnecting and naming](usage.md#reconnecting-and-naming) for details, or run
+`sbx ls` to see the names of your existing sandboxes.
+
 Installed packages, Docker images, and configuration changes are preserved
 across restarts. When you're done with a sandbox, remove it to reclaim disk
 space:
 
 ```console
-$ sbx rm my-sandbox
+$ sbx rm claude-my-project
 ```
 
 Removing a sandbox deletes everything inside it — installed packages, Docker