Feature request: Optional import syntax for CLAUDE.md

Problem

Shared project repos often reference external files via @path/to/file syntax in CLAUDE.md. When collaborators don't have these files (e.g., personal dotfiles at ~/Developer/dotfiles/profiles/vue.md), the import fails.

This prevents a useful pattern: maintaining a single source of truth for rules/profiles that multiple projects can reference, while still working for collaborators who don't have the external files.

Proposed Solution

Add optional import syntax that gracefully skips missing files:

@~/Developer/dotfiles/profiles/vue.md?

The trailing ? would indicate: "include if exists, skip silently if missing."

Use Case

• Personal dotfiles repo with frequently-updated rules/profiles
• Multiple shared project repos referencing these profiles
• Collaborators who clone the project don't have the dotfiles
• Current behavior: errors on missing files
• Desired behavior: graceful degradation (skip missing optional imports)