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/classify_test.go

@@ -0,0 +1,38 @@
+package main
+
+import "testing"
+
+// Every name in the common-rares set must classify as "common".
+func TestClassifyCommon(t *testing.T) {
+	if len(commonRares) != 74 {
+		t.Fatalf("expected 74 common rares, got %d", len(commonRares))
+	}
+	for name := range commonRares {
+		if got := classify(name); got != "common" {
+			t.Errorf("classify(%q) = %q, want common", name, got)
+		}
+	}
+}
+
+// Names not in the set (including near-misses) must classify as "great".
+func TestClassifyGreat(t *testing.T) {
+	greats := []string{
+		"Shimmering Skeleton Key",
+		"Star of Tukal",
+		"Hieroglyph of the Bludgeoner",
+		"Infinite Phial of Pyreal Flux",
+		"Foolproof Hooks",
+		"Staff of All Aphus",
+		"Count Renari's Equctioneer",
+		"Gelidite Long Sword",
+		"Pearl of Blade Baning ", // trailing space — not an exact match
+		"alchemist's crystal",    // wrong case — not an exact match
+		"Alchemist's Crystals",   // plural — not an exact match
+		"",
+	}
+	for _, name := range greats {
+		if got := classify(name); got != "great" {
+			t.Errorf("classify(%q) = %q, want great", name, got)
+		}
+	}
+}