fix(skills): avoid treating skill content as a template

[daninfpj]

Fixes #2654

Uses plain string replacement when substituting path placeholders in a skill's command field to avoid treating skill content as a Handlebars template, which would break skills containing {{ }} syntax (e.g. JSX or code examples).

Verification

• Lint and test pass
• Ran :ask command with the 12-principles-of-animation skill loaded
• Created skill with /create-skill and verified path replacement works correctly

[CLAassistant]

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