fix: validate custom install path in install.sh to prevent path traversal

[xiaolai]

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Security Fix (Low)

In examples/digital-brain-skill/scripts/install.sh, when the user selects option 3 (custom location), the input is read via read -p and passed directly to mkdir -p and cp -r without any validation:

read -p "Enter custom path: " custom_path
TARGET_DIR="$custom_path/$SKILL_NAME"

A user or script that passes a path containing .. segments could write files outside the intended directory. A path containing shell metacharacters could cause unexpected shell behavior depending on how the variable is expanded downstream.

Fix

Added validation after the read -p call that:

1. Rejects empty input
2. Rejects paths containing .. (traversal sequences)
3. Restricts characters to a safe allowlist: letters, numbers, /, _, ., -, ~, and spaces
4. Canonicalizes the path with realpath --canonicalize-missing to resolve any remaining traversal via symlinks or relative segments

if [[ -z "$custom_path" ]] || [[ "$custom_path" == *".."* ]] || [[ "$custom_path" =~ [^a-zA-Z0-9/_.\-\ ~] ]]; then
    echo "Invalid path. Use only letters, numbers, /, _, ., -, ~, and spaces."
    exit 1
fi
custom_path="$(realpath --canonicalize-missing -- "$custom_path")"

The built-in installation paths (options 1 and 2) are not affected since they are constructed from $HOME and . respectively, which are trusted values.