Phase D: presence intent comment + sendZoneCombatMessages contract

D1: single block comment above the presence.go best-effort QueryRow
cluster (lines 149-198). Zero-value fallbacks are correct for new users
with no activity rows yet.

D2: contract comment on sendZoneCombatMessages — returns a done channel,
callers chaining post-flush work MUST consume it; in-flow handlers with
nothing to chain MUST explicitly discard. Blocking on flush would stall
command handlers behind the 2-3s/message pacing, defeating streaming.
Two fire-and-forget sites (adventure_arena.go round-advance,
dnd_zone_cmd.go streamFlow) now use `_ =` + comment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/presence.go

@@ -146,6 +146,11 @@ func (p *PresencePlugin) handleWhois(ctx MessageContext) error {
 		displayName = string(targetUser)
 	}
 
+	// best-effort: activity tracking is non-critical; missing rows are
+	// expected for new users. The cluster of `_ = QueryRow(...).Scan(...)`
+	// calls below all share this rationale — zero values are the correct
+	// fallback (no messages yet, no streak, no rep, no presence row).
+
 	// Get stats
 	var totalMessages, totalWords int
 	_ = db.Get().QueryRow(