Clarify channel default skill missing Ornn setup

[louis4li]

Problem

When a channel registration has default_skill_name configured but the matching Ornn skill has not been created or published, the default-skill path fails through use_skill. The user-visible failure should make the setup problem explicit instead of looking like a generic skill load failure.

Acceptance Criteria

• For channel default-skill invocations where use_skill returns not_found, the final channel reply names the configured default skill and says it was not found in Ornn.
• The reply tells the operator to create/publish the same-named Ornn skill or update the channel registration default_skill_name.
• The default-skill path must not call ornn_search_skills or choose a replacement skill.
• Existing behavior for ordinary chat without default_skill_name remains unchanged.

Non-goals

• Do not automatically create or publish Ornn skills.
• Do not change sender binding requirements for explicit /skill or ::skill triggers.
• Do not add a second skill discovery path for channel defaults.