style: Params struct required even for single arguments

Only exception: stupidly obvious single args (featureflag.New(true)).
When in doubt, use Params.

prompts/CODE_STYLEGUIDE_GO.md

@@ -157,13 +157,14 @@ last_modified: 2026-02-22
     `server`. `util` is banned.
 
 1.  Constructors **must** take a `Params` struct (or `ThingParams` when
-    `NewThing()` is used). Positional arguments for constructors are an endless
+    `NewThing()` is used), even for a single argument. Named fields in a Params
-    source of bugs — they make call sites unreadable, invite wrong-order errors
+    struct are always clearer than positional arguments. Positional arguments
-    that the compiler can't catch when types coincide, and force every caller to
+    for constructors are an endless source of bugs — they make call sites
-    update when a new field is added. The only exception is a constructor that
+    unreadable, invite wrong-order errors that the compiler can't catch when
-    takes exactly one argument whose meaning is obvious from context (e.g.
+    types coincide, and force every caller to update when a new field is added.
-    `New(ctx)` or `FromBytes(b)`). Two or more arguments → use a Params struct,
+    The only exception is when the single argument is stupidly obvious from
-    no exceptions.
+    context — e.g. `featureflag.New(true)` or `thing.NewFromReader(r)`. When in
+    doubt, use a Params struct.
 
 1.  Use `context.Context` for all functions that need it. If you don't need it,
     you can pass `context.Background()`. Anything long-running should get and