fix(pstack): make a brand with a type guard, not as

pstack/skills/typescript-best-practices/SKILL.md

@@ -12,7 +12,7 @@ Apply the **type-system-discipline** principle skill first.
 | Rule | Summary |
 |------|---------|
 | Discriminated unions | Model variants with a `kind` literal discriminant so impossible states can't be represented. No optional-field bags. |
-| Branded types | Brand primitives with `& { readonly __brand: "X" }` so they can't be mixed up. Validate once at the boundary. |
+| Branded types | Brand primitives with `& { readonly __brand: "X" }` so they can't be mixed up. Validate once at the boundary. A brand comes from a verifying type guard (`input is AgentId`), never from `as`. |
 | Constructive modeling | Build the shape so the illegal value can't be constructed. `[T, ...T[]]` for non-empty, `[T, T][]` for even length, `start` plus `duration` for a range. Not a runtime guard, not a wish for refinement types. |
 | Simplest total type | Keep `T[]` while every operation on it stays total. Strengthen to `NonEmpty<T>` only where the loose type forces `!`, a cast, or a "should never happen" throw. |
 | `unknown` over `any` | External data is `unknown`. |

pstack/skills/typescript-best-practices/references/patterns.md

@@ -9,9 +9,13 @@ Brand primitives so they can't be mixed up. Validate once at the boundary. Downs
 ```ts
 type AgentId = string & { readonly __brand: "AgentId" };
 
+function isAgentId(input: string): input is AgentId {
+  return isUUID(input);
+}
+
 function parseAgentId(input: string): AgentId {
-  if (!isUUID(input)) throw new Error(`Invalid agent id: ${input}`);
-  return input as AgentId;
+  if (!isAgentId(input)) throw new Error(`Invalid agent id: ${input}`);
+  return input;
 }
 
 function focusAgent(id: AgentId): void {