Fix mktemp template so deploy-managed-agent.sh runs on Linux

GNU mktemp wants 3+ X's in a -t template. The bare `mktemp -t skillcache`
and `mktemp -t skill` calls worked on macOS but crash on Linux before
the script does anything. test-cookbooks.sh fails the same way for all
5 cookbooks because it calls deploy --dry-run.

Adding .XXXXXX is portable: GNU expands the X's, BSD treats them as
prefix. Confirmed on Ubuntu 24.04 / coreutils 9.4: test-cookbooks goes
0/5 -> 5/5.

scripts/deploy-managed-agent.sh

@@ -64,7 +64,7 @@ json.dump(yaml.safe_load(t), sys.stdout)
 ' "$1"
 }
 
-SKILL_CACHE_FILE="$(mktemp -t skillcache)"
+SKILL_CACHE_FILE="$(mktemp -t skillcache.XXXXXX)"
 trap 'rm -f "$SKILL_CACHE_FILE"' EXIT
 upload_skill() {
   local path="$1" key cached
@@ -77,7 +77,7 @@ upload_skill() {
     printf '%s' "$cached"; return
   fi
   local resp id zip
-  zip="$(mktemp -t skill).zip"
+  zip="$(mktemp -t skill.XXXXXX).zip"
   (cd "$(dirname "$path")" && zip -qr "$zip" "$(basename "$path")")
   # /v1/skills uses its own beta header and multipart, not the managed-agents JSON path
   resp=$(curl -sS "$API/v1/skills" \