refactor(perf): Improve robustness and performance

- [add] Create safeQuerySelector helper function (github-commit-labels.js:1203-1211)
- [add] Implement debounce utility function (github-commit-labels.js:1214-1225)
- [refactor] Use multiple selectors for finding commit messages (github-commit-labels.js:1239-1252)
- [add] Add try/catch error handling in commit processing loop (github-commit-labels.js:1263-1414)
- [perf] Apply debounce to MutationObserver callbacks (github-commit-labels.js:1436-1449)
- [refactor] Create dedicated setupMutationObserver function (github-commit-labels.js:1435-1451)
- [perf] Add cleanup listener on page unload (github-commit-labels.js:1462-1464)
- [perf] Apply debounce to navigation and event handlers (github-commit-labels.js:1470-1503)

Author: nazdridoy