[FEATURE] Enable ShellCheck require-variable-braces optional check

[WilliamBerryiii]

Problem Statement

The codebase has inconsistent variable reference formatting - some use $var while others use ${var}. ShellCheck's SC2250 optional check can enforce the braced format (${var}) for consistency. Initial analysis shows 3,145 potential changes across 81 scripts.

Proposed Solution

Enable SC2250 in .shellcheckrc and remediate existing scripts:

1. Add enable=require-variable-braces to .shellcheckrc
2. Create automated script to convert $var to ${var}
3. Run conversion across all shell scripts
4. Validate no functional changes

Alternative Solutions

• Gradual adoption: Enable SC2250 but suppress existing violations, fix incrementally (slow, defeats purpose)
• Manual conversion: Fix each script by hand (time-consuming, error-prone)
• Do nothing: Accept inconsistent formatting (status quo)

Target Components

• Terraform modules
• Bicep modules
• Blueprints
• GitHub Actions
• Documentation
• Other: Shell scripts (81 files with 3,145 potential changes)

Implementation Ideas

Discovery command used:

shellcheck --enable=require-variable-braces --format=gcc *.sh 2>&1 | grep -c SC2250

Breakdown by directory:

Directory	SC2250 Warnings
scripts/	2,847
src/100-edge/100-cncf-cluster/scripts/	113
src/starter-kit/*/scripts/	142
Other directories	43

Automated fix approach:

• Use sed or custom script to convert $var → ${var}
• Exclude special variables: $@, $*, $?, $!, $$, $#, positional $1-$9
• Preserve quoted contexts

Additional Context

• Parent issue: [FEATURE] Add .shellcheckrc for ShellCheck configuration #116 (ShellCheck configuration)
• SC2250 is an OPTIONAL check (must be explicitly enabled)
• High volume: 3,145 warnings across 81 scripts
• Estimated effort: 2-4 hours (mostly automated conversion + validation)
• Recommend: Complete Issues [FEATURE] Remove redundant SC1091 inline shellcheck directives #125 and [FEATURE] Add ShellCheck source-path directive for relative script sourcing #126 first due to lower effort

Benefits

• Consistent variable formatting across all scripts
• Improved readability and maintainability
• Eliminates ambiguity in variable boundaries (e.g., ${var}suffix vs $varsuffix)
• Aligns with shell scripting best practices

Potential Challenges

• High volume of changes requires thorough testing
• Automated conversion must be carefully validated
• Some edge cases may need manual review
• Large PR may be difficult to review