fix: shellcheck errors in start.sh; add code-quality CI workflow

[genui-scotty]

Summary

🐛 start.sh — ShellCheck fixes

• SC2004 (line 94): removed unnecessary $ on arithmetic variable — $charwidth → charwidth
• SC1083 (line 123): quoted curl format string — %{http_code} → '%{http_code}'
• SC2154 (line 143): fixed typo — ${chars:$iterator:1} → ${char:$iterator:1} (variable chars was never defined; char was)

✅ .github/workflows/code-quality.yml

Adds a CI workflow that runs on PRs to main, following the GenUI pattern:

• Detects changed markdown and shell files via tj-actions/changed-files@v47
• Lints markdown with DavidAnson/markdownlint-cli2-action@v22
• Lints shell scripts with reviewdog/action-shellcheck@v1
• Short-circuits cleanly when no relevant files changed

📄 .github/workflows/CODE_QUALITY.md

Documents the workflow — what runs, when, how to lint locally, and shellcheck suppressions.

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Verified: shellcheck = 0 errors, markdownlint-cli2 = 0 errors before commit.