docs: guide Flashduty large-list scans

[ysyneu]

Summary

• Add Flashduty skill guidance to redirect page-sized JSON/list scans to temp files and summarize selected fields only.
• Add incident-summary reuse guidance to avoid redundant incident subquery fetches.
• Add a skilldoc test that guards the guidance in the CLI-owned Flashduty skill source.

Audit Context

Automation: fc-safari-audit-session
Run: audit-20260709T031722Z-auto
Window: 2026-07-08T03:15:34.438Z -> 2026-07-09T03:17:22.000Z

Safari embeds this skill, but the command-card source of truth lives here, so this companion PR keeps the source guidance explicit.

Verification

• go test ./internal/skilldoc
• make check-cards
• go test -race ./...
• go build -v -ldflags "-s -w ..." -o bin/flashduty ./cmd/flashduty
• committed branch review: no blocking findings
• ./bin/golangci-lint run remains blocked in this local environment because the available golangci-lint reports it was built with Go 1.24 while the repo targets Go 1.25.1; rebuilding via go install in this shell produced the same embedded Go version.