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 ✅.

cmd/plexum-fabric-channel-plugin/main.go

@@ -29,6 +29,7 @@ import (
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/fabric"
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/identity"
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/inbound"
+	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/presence"
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/subdisc"
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/tokens"
 	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/tools"
@@ -43,9 +44,10 @@ import (
 //	  "sync_commands": ["new","stop"]  // optional override; defaults to ["new","stop"]
 //	}
 type HostConfig struct {
-	CenterAPIBase   string   `json:"center_api_base"`
-	CommandsSyncKey string   `json:"commands_sync_key,omitempty"`
-	SyncCommands    []string `json:"sync_commands,omitempty"`
+	CenterAPIBase           string   `json:"center_api_base"`
+	CommandsSyncKey         string   `json:"commands_sync_key,omitempty"`
+	SyncCommands            []string `json:"sync_commands,omitempty"`
+	PresenceIntervalSeconds int      `json:"presence_interval_seconds,omitempty"` // F-5b; 0 → default 30s
 }
 
 type fabricPlugin struct {
@@ -237,6 +239,21 @@ func (p *fabricPlugin) Init(ctx context.Context, host plugin.HostAPI) error {
 		}()
 		host.Log("info", "fabric inbound supervisor started",
 			map[string]any{"agents": sup.AgentIDs, "bindings": len(p.bindings)})
+
+		// F-5b: presence-sync. Shares the inbound supervisor's context
+		// so it dies together. Tick interval honours config override.
+		interval := time.Duration(p.cfg.PresenceIntervalSeconds) * time.Second
+		ps := &presence.Sync{
+			Host:       host,
+			Client:     p.client,
+			Tokens:     p.tokens,
+			Identities: p.identities,
+			Bindings:   p.bindings,
+			Interval:   interval,
+		}
+		ps.Start(ctxBg)
+		host.Log("info", "fabric presence-sync started",
+			map[string]any{"interval": ps.Interval.String()})
 	}
 
 	return nil