Keep the stored GitHub token when setup runs again

The secret store outlives the sandbox, so recreating one to change its image
or add a profile does not lose the token. Prompting for a replacement anyway
made --replace impractical and trained the habit of minting tokens that are
never revoked.

Only a token scoped to this sandbox counts. A global one would authenticate
the agent too, but reaching every repository it can reach is what this task
exists to prevent. The token command still always prompts; it is the way to
replace an expired or revoked token.

tasks/ai/sbx

@@ -83,8 +83,10 @@ Usage:
   mise run ai:sbx -- status
   mise run ai:sbx -- remove
 
-Setup opens a pre-filled GitHub token form in your browser. Use "token" on
-its own to replace an expired or revoked token later.
+Setup opens a pre-filled GitHub token form in your browser, unless a token is
+already stored for the sandbox. The secret store outlives the sandbox, so
+recreating one with --replace keeps its token. Use "token" on its own to
+replace an expired or revoked token.
 
 Set AI_SBX_TEMPLATE in your mise config to reuse one custom image across
 every repository without repeating --template.
@@ -242,6 +244,17 @@ read_token() {
     printf '%s' "$token"
 }
 
+# Only a token scoped to this sandbox counts. A global one would authenticate
+# the agent too, but reaching every repository the token can reach is exactly
+# what this task exists to prevent, so it is not treated as satisfying setup.
+sandbox_has_github_token() {
+    sbx secret ls 2>/dev/null |
+        awk -v scope="$SANDBOX_NAME" '
+            $1 == scope && $2 == "service" && $3 == "github" { found = 1 }
+            END { exit !found }
+        '
+}
+
 install_github_token() {
     local url
     url="$(token_url)"
@@ -1354,7 +1367,15 @@ setup_command() {
         create_sandbox
     fi
 
-    install_github_token
+    # The secret store outlives the sandbox, so recreating one to change its
+    # image keeps the token. Prompting anyway would train the habit of minting
+    # replacement tokens and never revoking the old ones.
+    if sandbox_has_github_token; then
+        printf 'Keeping the GitHub token already stored for %s. Replace it with: mise run ai:sbx -- token\n' \
+            "$SANDBOX_NAME"
+    else
+        install_github_token
+    fi
 
     install_sandbox_aws_files