fix(command): quote argument-hint so cost.md YAML frontmatter parses

Unquoted `argument-hint: [filter] ...` starts with `[`, which YAML reads as
a malformed flow sequence, so Claude Code silently drops ALL of the command's
frontmatter at runtime (description, argument-hint, allowed-tools). Quoting the
value fixes it. Caught by `claude plugin validate` on the plugin manifest (the
marketplace-manifest validation had masked it). Plugin + marketplace both
validate clean now.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

Author: solomonic

commands/cost.md

@@ -1,6 +1,6 @@
 ---
 description: Show your Claude Code spend in USD per project and per git branch, computed locally from the session logs Claude Code already writes. Never touches API traffic; zero key, zero prompts, zero latency.
-argument-hint: [filter] [--self] [--json]
+argument-hint: "[filter] [--self] [--json]"
 allowed-tools: Bash(node:*), Bash(budgetclaw:*)
 ---