test: add coverage for skill session/runtime/context branches

[fennoai]

Requested by @Yumiue

Summary

• add branch tests for session skill activation clone/blank input paths
• add runtime skill API and state helper branch coverage tests
• add context skill prompt helper branch coverage tests
• add bootstrap skills registry refresh-failure fallback test

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 90.08043% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/runtime/skills.go	82.05%	12 Missing and 9 partials ⚠️
internal/context/source_skills.go	90.90%	5 Missing and 5 partials ⚠️
internal/session/skill_activation.go	91.66%	3 Missing and 3 partials ⚠️

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