Add more scripts for closing-obsolete-issues skill

[srawlins]

I was running with an agent using the 'close-obsolete-issues' skill and it kept making ad-hoc python scripts; I asked it to check them in as Dart scripts. This should make this task faster in the future.

[gemini-code-assist]

Code Review

This pull request replaces shell scripts with new Dart scripts (fetch_issues.dart and search_prs.dart) for fetching issues and searching PRs, and updates the SKILL.md documentation accordingly. The review feedback highlights a contradictory style constraint in SKILL.md caused by an accidental character replacement, and suggests robustness improvements in the Dart scripts, including validating command-line argument parsing, using idiomatic list pre-allocation, and handling potential JSON decoding exceptions.

[gemini-code-assist]

[NIT] Setting the length property of a growable list directly is discouraged in Dart. It is more idiomatic and safer to use List.filled to pre-allocate the list.

Suggested change

	final results = <Map<String, dynamic>?>[];
	results.length = numbers.length;
	final results = List<Map<String, dynamic>?>.filled(numbers.length, null);

References

1. Adhering to consistent style and best practices across all DevTools packages improves collaboration and reduces errors. ^(link)