Add b2 skill: Backblaze B2 management CLI (storage cost, prefix purge)

B2 Native API v3 client for the ACG B2 account: status, buckets, keys,
files, bucket-size, usage/cost ($0.00695/GB), gated create/delete bucket+key,
and gated lifecycle-based delete-prefix/lifecycle-remove for prefix purges.
Read-only by default; destructive ops require --confirm.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/skills/b2/.gitignore

@@ -0,0 +1,3 @@
+# b2 skill local cache. Holds the live B2 authorization token (a SECRET, valid
+# ~24h) plus the per-account apiUrl/accountId. NEVER commit this directory.
+.cache/