Clarify missing channel default skill setup

[louis4li]

Issue

Closes #3628

Scope

• Add channel-default-skill-specific setup guidance when use_skill cannot load the configured skill from Ornn.
• Preserve default-skill routing behavior: no Ornn search or replacement skill selection for the default binding path.
• Cover the runner prompt and use_skill not-found result with focused tests.

Verification

• dotnet test test/Aevatar.GAgents.ChannelRuntime.Tests/Aevatar.GAgents.ChannelRuntime.Tests.csproj --nologo --filter "FullyQualifiedName~ChannelConversationTurnRunnerTests" passed: 167 passed, 0 failed, 0 skipped.
• dotnet test test/Aevatar.AI.ToolProviders.Ornn.Tests/Aevatar.AI.ToolProviders.Ornn.Tests.csproj --nologo --filter "FullyQualifiedName~LocalSkillCatalogTests" passed: 17 passed, 0 failed, 0 skipped.
• bash tools/ci/test_stability_guards.sh passed.
• git diff --check passed.
• Local NyxID/API endpoint check not run; this change is limited to prompt/tool-result behavior covered by focused unit tests.

Notes

• Existing restore/build warnings were present during test runs and did not fail verification.

Generated with Claude Code.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-15T04:23:51.552840Z	4e2b661	PR opened

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Reviewed commit: 4e2b66121f

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