refactor(builder): eliminate code smells in builder module

[flexiondotorg]

• Remove dead Enabled() helper function from pointer.go
• Fix resource leak: move ticker.Stop() outside retry loop in download.go
• Flatten single-element slice loop in state.go
• Replace magic string sentinels with pointer identity checks in libraries.go
• Extract duplicated SDK path extraction logic into extractSDKPath() helper

Checklist

• I have performed a self-review of my code
• I have tested my changes and confirmed there are no regressions

[cubic-dev-ai]

No issues found across 6 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: Refactor touches core logic across multiple files, including resource management in download.go and library ordering in libraries.go, which require human verification.}