[FEATURE] Support substituting ${CLAUDE_PROJECT_DIR} in skills.

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

While skills allow substituting the skill directory, they don't currently have a substitution variable for substituting the project directory.

Proposed Solution

Add ${CLAUDE_PROJECT_DIR} to the set of variables that are substituted in the skill.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

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Additional Context

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