feat: track karma channel and global totals

[variablenix]

Summary

• track Karma per network/channel alongside the existing global total
• show screenshot-style channel and global context in channel updates and !karma
• preserve legacy global Karma values while new channel totals start at zero
• update global and channel values atomically in one Bolt transaction
• add regression coverage for formatting, persistence, migration, and network isolation
• document the new scope behavior

Example channel update:
🆙 Karma boost! project gained 1 karma ✨ 🌟 💫 (🎯 9 in #chat | 🌐 19 global)

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• go mod verify
• git diff --check