Add test session investigation skill flow

[thebiglabasky]

Summary

Adds a dedicated npx checkly skills investigate test-sessions reference for the recently added test-session investigation flow.

The new reference covers:

• running or triggering recorded sessions
• listing and inspecting test sessions, including --watch
• drilling into test-session error groups
• reusing existing RCA or running rca run --test-session-error-group
• retrieving logs/traces/videos/screenshots/assets from JSON result payloads without implying a nonexistent download command

The high-level generated skill summary now also points agents directly at npx checkly skills investigate test-sessions, and the top-level investigate action description mentions test sessions.

Why

The existing investigate skill only pointed agents at deployed check inspection. Agents did not have a clear progressive-disclosure path for session-first debugging, test-session error groups, RCA handoff, or result asset lookup.

Validation

• pnpm --filter checkly run prepare:dist
• pnpm --filter checkly exec jiti scripts/prepare-ai-context.ts
• pnpm run sync:skills
• pnpm --filter checkly exec vitest --run command-metadata
• ./packages/cli/bin/run skills investigate test-sessions
• ./packages/cli/bin/run skills

Note: prepare-ai-context emitted existing warnings about missing generated example files under packages/cli/gen, but completed and generated the new reference successfully.