fix(media-use): forgiving prompt matching + precise assets/ scan

Two defects in the resolve cascade that made cache and asset-reuse
misbehave in practice:

- Prompt matching was byte-exact and case-sensitive. findByPrompt and
  cacheGet compared provenance.prompt with ===, so "Calm piano" and
  "calm  piano" re-searched and re-downloaded instead of reusing the
  cached asset (same project and cross-project). Add normalizePrompt
  (trim + lowercase + collapse whitespace) and key both lookups on it;
  the raw prompt is still stored for audit.

- findExistingAsset matched with name.includes(intent) ||
  intent.includes(name), which silently returned the WRONG local file:
  intent "whoosh" grabbed a stray who.mp3, and a one-letter filename
  matched every intent. Require a shared word token (>= 3 chars, minus
  stopwords) so a false negative just falls through to a catalog search
  rather than shipping the wrong asset.

Adds lib/adopt.test.mjs and extends manifest.test.mjs. Full media-use
suite green; verified e2e against the live catalog (case-variant
cross-project resolve now reuses; whoosh no longer grabs who.mp3).

skills/media-use/scripts/lib/cache.mjs

@@ -2,7 +2,7 @@ import { readFileSync, writeFileSync, mkdirSync, existsSync, copyFileSync } from
 import { join, basename } from "node:path";
 import { createHash } from "node:crypto";
 import { homedir } from "node:os";
-import { readManifest, appendRecord } from "./manifest.mjs";
+import { readManifest, appendRecord, normalizePrompt } from "./manifest.mjs";
 
 const SCHEMA_PREFIX = "mu-v1-";
 const KEY_HEX_CHARS = 16;
@@ -43,9 +43,14 @@ function validateCacheHit(match) {
 }
 
 export function cacheGet(prompt, type) {
+  const key = normalizePrompt(prompt);
+  if (!key) return null;
   return validateCacheHit(
     readGlobalManifest().find(
-      (r) => r.reusable && r.provenance?.prompt === prompt && (type == null || r.type === type),
+      (r) =>
+        r.reusable &&
+        normalizePrompt(r.provenance?.prompt) === key &&
+        (type == null || r.type === type),
     ),
   );
 }