docs(investigate): distinguish issues from stumbles

[rsd-raindrop]

Summary

• define Issue Detection reports as broader changes in an organization's event distribution
• define Stumbles as one-off unique failure modes from individual interactions
• teach broad investigations such as "what issues happened today?" to query both catalogs independently
• document the additive search_stumbles MCP tool and refresh the list_issues contract

Rollout

• public skill sync for invisible-tools/dawn#3536
• merge after the Dawn MCP implementation is live