Simplify ts-core auth.ts; fix bun-types resolution in monorepo workspace

auth.ts: dropped the legacy HS256-shared-secret verification path, keeping
only the JWKS path Supabase recommends now — one code path instead of two.

Also fixes a real bug: @types/bun's ambient types didn't resolve inside a
bun workspace (its internal 'bun-types' reference can't hoist into a nested
package's node_modules the same way it does in a flat install). Depending on
bun-types directly instead of @types/bun fixes it — verified with a clean
'bun run typecheck'.

packages/ts-core/tsconfig.json

@@ -4,6 +4,7 @@
     "module": "ESNext",
     "moduleResolution": "Bundler",
     "lib": ["ES2022"],
+    "types": ["bun-types"],
     "strict": true,
     "declaration": true,
     "declarationMap": true,