sbx: fix broken commit-signing anchor links

Two pages linked to usage.md#signed-commits, an anchor that does not
exist. The commit-signing content lives in workflows.md under the
"Commit signing" heading. Point both links there.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

content/manuals/ai/sandboxes/security/credentials.md

@@ -198,7 +198,7 @@ Use SSH agent forwarding for Git operations over SSH and SSH-based commit
 signing. The signing key must be loaded in the host SSH agent for sandboxed
 commit signing to work. Outbound SSH connections are still subject to sandbox
 network policy. For details, see
-[Signed commits](../usage.md#signed-commits).
+[Commit signing](../workflows.md#commit-signing).
 
 ## Custom secrets
 

content/manuals/ai/sandboxes/troubleshooting.md

@@ -241,7 +241,7 @@ $ git branch -D <branch-name>
 ## Sandbox commits aren't signed
 
 Docker Sandboxes can sign Git commits with SSH keys from your host agent.
-For setup steps, see [Signed commits](usage.md#signed-commits).
+For setup steps, see [Commit signing](workflows.md#commit-signing).
 
 If `ssh-add -L` prints `The agent has no identities.`, the sandbox can reach
 the forwarded agent, but the host agent doesn't have a loaded key. Load the