refactor: use #shadow/env for replicant dev-http port instead of rewriting config

Replace the pattern of reading shadow-cljs.edn, mutating it with
pr-str (which destroyed formatting), and writing it back. Instead,
use shadow-cljs's built-in `#shadow/env` reader tag to read the port
from the SHADOW_HTTP_PORT environment variable at startup, with a
default of 3001.

Both `dev-replicant` and `dev-all` tasks now pass the port via env var
instead of file mutation. This eliminates the dirty-file problem where
the rewritten single-line config would show up as an unrelated change.

dev/replicant/shadow-cljs.edn

@@ -1,5 +1,5 @@
 {:deps true
- :dev-http {3001 "public"}
+ :dev-http {#shadow/env ["SHADOW_HTTP_PORT" :as :int :default 3001] "public"}
  :builds
  {:app
   {:target :browser