fix(policy): restore Policy typedef import in policy-validators (addresses Jonathan review)

In ccc1b5e the `Policy` typedef import in policy-validators.js was
dropped alongside the dead `collectReferencedOperations` function — but
`validatePolicy` still uses `@param {Policy}`. The hand-edited
`policy-validators.d.ts` carries `export type Policy = …`, but a fresh
`npm run build:types` (pinned tsc 5.8.3) regenerates it without that
export, leaving the param type unresolved.

Re-adding the typedef makes `build:types` reproduce the committed
`policy-validators.d.ts` byte-for-byte (verified locally).

Catch by jonathunne:
#55 (comment)

Author: AlonzoRicardo

src/policy/policy-validators.js

@@ -24,6 +24,7 @@ import {
   registerOptionsSchema
 } from './policy-schemas.js'
 
+/** @typedef {import('./policy-engine.js').Policy} Policy */
 /** @typedef {import('./policy-engine.js').PolicyRule} PolicyRule */
 /** @typedef {import('./policy-engine.js').RegisterPolicyOptions} RegisterPolicyOptions */