Persist plugin usage metrics for Grafana

[variablenix]

Summary

• persist per-network plugin command totals in BoltDB
• restore plugin metrics after GoBot restarts so Most-used plugins remains populated
• scope the Prometheus scrape panel to targets exposing GoBot metrics when filters are All
• add restart regression coverage and update monitoring documentation

Verification

• go test ./...
• go vet ./...
• go test -race ./...
• go build -trimpath -o /tmp/gobot-irc-bot ./cmd/irc-bot
• go run golang.org/x/vuln/cmd/govulncheck@latest ./...