fix: quote argument-hint values to prevent YAML object parsing

Unquoted bracket syntax in `argument-hint` frontmatter causes YAML to
parse the value as an array/mapping instead of a string literal. This
crashes Claude Code's tab-completion TUI with React error #31 ("Objects
are not valid as a React child") when the renderer tries to display the
hint.

Two commands affected:
- `heal-skill`: `[optional: ...]` parsed as `[{optional: "..."}]`
- `create-agent-skill`: `[skill ...]` parsed as `["skill ..."]`

Fix: wrap values in quotes, consistent with the other 18 commands in the
plugin that already quote their `argument-hint` values.

Ref: https://github.com/anthropics/claude-code/issues/29422

plugins/compound-engineering/commands/create-agent-skill.md

@@ -2,7 +2,7 @@
 name: create-agent-skill
 description: Create or edit Claude Code skills with expert guidance on structure and best practices
 allowed-tools: Skill(create-agent-skills)
-argument-hint: [skill description or requirements]
+argument-hint: "[skill description or requirements]"
 disable-model-invocation: true
 ---
 

plugins/compound-engineering/commands/heal-skill.md

@@ -1,7 +1,7 @@
 ---
 name: heal-skill
 description: Fix incorrect SKILL.md files when a skill has wrong instructions or outdated API references
-argument-hint: [optional: specific issue to fix]
+argument-hint: "[optional: specific issue to fix]"
 allowed-tools: [Read, Edit, Bash(ls:*), Bash(git:*)]
 disable-model-invocation: true
 ---