feat(presence): F-5b presence-sync — mirror sm.Machine into Fabric

Adds an internal/presence package that ticks every 30s (configurable
via presence_interval_seconds), reads each bound agent's sm.Machine
state through host.ReadAgentState, maps to Fabric's 6-status enum,
and PUTs diffs to /api/agents/:userId/presence on every guild the
agent belongs to.

Semantic mapping (the part flagged "needed" in the prior README):
  idle    → idle
  working → on_call
  busy    → busy
  offline → offline
exhausted/unknown reserved for backend-side fallbacks; we don't push.

Tick is mutex-guarded (avoids the upsert race the openclaw incident
called out in agent-presence.service.ts) and diff-gated so writes are
sparse. Token-cache invalidation on PUT failure handles guild JWT
rotation.

fabric.Client gains SetAgentPresence helper. README marks F-5b ✅.

internal/presence/presence_test.go

@@ -0,0 +1,21 @@
+package presence
+
+import "testing"
+
+func TestStateToFabricMapping(t *testing.T) {
+	cases := []struct {
+		in, want string
+	}{
+		{"idle", "idle"},
+		{"working", "on_call"},
+		{"busy", "busy"},
+		{"offline", "offline"},
+		{"", ""},
+		{"weird", ""},
+	}
+	for _, c := range cases {
+		if got := StateToFabric(c.in); got != c.want {
+			t.Errorf("StateToFabric(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}