perf(conf.d): skip starship.fish in non-interactive shells

Defines fish_prompt only. The guard precedes the op-guard and the type -q
PATH scan so both are skipped in scripts. Scripts fall back to the repo's
autoloadable functions/fish_prompt.fish, which nothing invokes anyway.

conf.d/starship.fish

@@ -9,6 +9,10 @@
 # Without starship, fish's built-in prompt already emits OSC 133;A
 # on the prompt line itself, so no wrapper is needed.
 
+# Defines fish_prompt; no script renders a prompt. Checked before the
+# op-guard so the builtin short-circuits ahead of three function autoloads.
+status is-interactive; or return
+
 # Replacing the prompt is opinionated (C3 overrides)
 __fish_config_op_enabled (status basename); or return