perf: cache loaded templates in memory

[Darkroom4364]

• Module-level cache avoids re-reading YAML on every call
• clearTemplateCache() export for tests

[Darkroom4364]

Scope: Only loader.ts + index.ts touched — confirmed, clean.

Cache logic: Correct. The cache stores the full unfiltered list; depth filtering is applied on read via .filter(), so a depth=passive call doesn't poison the cache for later depth=active calls. loadTemplateById benefits automatically since it calls loadTemplates().

One nit: loadTemplates returns _cache directly (not a copy). Any caller mutating the returned array (push/splice) would corrupt the cache. Returning [..._cache] (shallow copy) would be safer, but fine to skip if callers are read-only today.

clearTemplateCache is exported from both loader.ts and index.ts. LGTM.