feat(go-services): discord-go — Go port of discord-rare-monitor

Consumes the Python tracker's /ws/live firehose (subscribed to rare+chat),
classifies rares common/great, posts embeds + relays allegiance chat.

- classify.go: the 74-name common-rares set, extracted verbatim from the Python
  COMMON_RARES_PATTERN (not hand-transcribed). go test runs at build time; a
  server-side dump-rares vs the Python regex confirms the sets are IDENTICAL.
- poster.go: a `poster` interface with a real discordgo impl (REST sends by
  channel id; gold/blue embeds, location/time fields, icon attachment) and a
  dry-run log impl.
- ws.go: coder/websocket client to /ws/live with subscribe, ping keepalive,
  exponential-backoff reconnect; rare/chat dispatch incl. vortex-warning + the
  MONITOR_CHARACTER filter.
- SAFE BY DEFAULT: dry-run unless a token AND DRY_RUN=0 are set, so it can never
  double-post to production. Deployed via the compose override
  (discord-rare-monitor-go), running dry-run against the same live firehose.

Validated on the server: connects, subscribes, relays a real chat in dry-run;
classifier parity 74/74 vs the Python regex.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go-services/discord-go/main.go

@@ -0,0 +1,90 @@
+// Command discord-go is a Go port of discord-rare-monitor: it consumes the
+// tracker's /ws/live firehose (subscribed to rare+chat), classifies rares
+// common/great, posts embeds to Discord, and relays allegiance chat.
+//
+// SAFETY: it runs in dry-run (log only, no Discord posts) by default. Going live
+// requires BOTH a bot token AND DRY_RUN=0 — so it can never accidentally
+// double-post to the production channels during the parallel run. For a parallel
+// test, set a TEST token + TEST channel IDs + DRY_RUN=0.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/signal"
+	"sort"
+	"syscall"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+func main() {
+	// `discord-go dump-rares` prints the common-rares set (for parity diffing
+	// against the Python regex). No network, no token.
+	if len(os.Args) > 1 && os.Args[1] == "dump-rares" {
+		names := make([]string, 0, len(commonRares))
+		for n := range commonRares {
+			names = append(names, n)
+		}
+		sort.Strings(names)
+		for _, n := range names {
+			fmt.Println(n)
+		}
+		return
+	}
+
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))
+	slog.SetDefault(logger)
+
+	token := os.Getenv("DISCORD_RARE_BOT_TOKEN")
+	// Dry-run unless a token is present AND DRY_RUN is explicitly "0".
+	dryRun := token == "" || os.Getenv("DRY_RUN") != "0"
+
+	wsURL := envOr("DERETH_TRACKER_WS_URL", "ws://dereth-tracker:8765/ws/live")
+	monitorChar := envOr("MONITOR_CHARACTER", "Dunking Rares")
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	var out poster
+	if dryRun {
+		reason := "no DISCORD_RARE_BOT_TOKEN"
+		if token != "" {
+			reason = "DRY_RUN != 0"
+		}
+		logger.Info("starting in DRY-RUN — classifying but NOT posting to Discord", "reason", reason, "ws", wsURL, "monitor", monitorChar)
+		out = &logPoster{log: logger}
+	} else {
+		dg, err := discordgo.New("Bot " + token)
+		if err != nil {
+			logger.Error("discord session init failed", "err", err)
+			os.Exit(1)
+		}
+		// REST-only: we send by channel ID, so no gateway Open()/intents needed.
+		logger.Info("starting LIVE — posting to Discord", "ws", wsURL, "monitor", monitorChar)
+		out = &discordPoster{
+			dg:       dg,
+			common:   envOr("COMMON_RARE_CHANNEL_ID", "1355328792184226014"),
+			great:    envOr("GREAT_RARE_CHANNEL_ID", "1353676584334131211"),
+			aclog:    envOr("ACLOG_CHANNEL_ID", "1349649482786275328"),
+			sawato:   envOr("SAWATOLIFE_CHANNEL_ID", "1387323032271327423"),
+			iconsDir: envOr("ICONS_DIR", "icons"),
+			log:      logger,
+		}
+	}
+
+	b := &bot{wsURL: wsURL, monitorChar: monitorChar, out: out, log: logger}
+	go b.run(ctx)
+
+	<-ctx.Done()
+	logger.Info("shutdown signal received")
+}
+
+func envOr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}