Extract duplicated `SkillBase` helpers

[cassieemb]

Summary

Several builtins duplicate the same boilerplate.

Where

skills/builtin/web_search.rb, datasphere.rb, weather_api.rb, wikipedia_search.rb (the *_BASE_URL env-override + trailing-slash-strip block, and @api_key && !@api_key.empty?-style guards); weather_api.rb hand-rolls a tool hash that joke.rb builds via DataMap.

Directive

Add helpers on SkillBase — e.g. resolved_base_url(env_var:, default:), a blank-check helper, and a prompt-section builder — and use them across the builtins; switch weather_api to DataMap.

Guardrails

Keep each skill's emitted tool/SWML output unchanged.

Acceptance

• the duplicated blocks are gone
• skill outputs unchanged
• rake test green

Verify

rake test; diff emitted SWML for an example using each touched skill before/after.