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>

.gitignore

@@ -8,6 +8,9 @@ tmp-remediation/
 # Bitdefender skill cache (identity/structure only — no secrets/PII)
 .claude/skills/bitdefender/.cache/
 
+# B2 skill cache (holds the live B2 authorization token — a SECRET)
+.claude/skills/b2/.cache/
+
 # Local settings (machine-specific)
 .claude/settings.local.json
 .claude/identity.json