feat: Phase F-1 — Plexum-fabric-channel-plugin foundation

Ports the foundation of Fabric.OpenclawPlugin to a native Plexum
channel plugin (Go). F-2+ phases (socket.io inbound, wakeup gate,
tools, presence, etc.) follow.

Layout:
  internal/identity/      — fabric-identity.json registry (agent → API key)
  internal/fabric/        — REST client (Center auth + Guild messaging)
  internal/config/        — channels/<name>.json fabric extension parser
  cmd/plexum-fabric-register/      — agent registration CLI
  cmd/plexum-fabric-channel-plugin/— Plexum SDK plugin entry
  scripts/install.sh      — build + install + manifest generator

Plugin behavior (F-1):
- Reads <profile>/channels/*.json, filters plugin=plexum-fabric-channel,
  builds (plexum-channel-name → fabric channel-id) index
- Validates each bound agent's API key against Center at init
  (warmSessions); logs warning but doesn't refuse init on bad keys
- `send` MCP tool: POST plain text to the bound Fabric channel as the
  agent user; selects guild endpoint+token from cached session
- Manifest channels[] is generated by install.sh from current
  channels/*.json — re-run with --reset-manifest after adding bindings
- Plugin-private config at
  <profile>/plugins/plexum-fabric-channel/config.json
  (center_api_base, default http://localhost:7001/api)

Live smoke verified:
- plexum-fabric-register against running Fabric Center (port 7001):
  validated fak_..., wrote identity file with user_id + email captured

Tests: identity (5) + config (6) = 11 unit tests.

F-2 will hook socket.io for inbound + wakeup gating + token refresh.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/plexum-fabric-register/main.go

@@ -0,0 +1,91 @@
+// Command plexum-fabric-register binds a Plexum agent to a Fabric
+// Center API key. Equivalent of openclaw's `fabric-register` script.
+//
+// Usage:
+//
+//	plexum-fabric-register --api-key fak_...
+//	    # agent id from $AGENT_ID env (set by exec MCP tool)
+//	plexum-fabric-register --agent-id alice --api-key fak_...
+//
+// Flags:
+//   --api-key    Required. The Center-issued agent API key.
+//   --agent-id   Optional when $AGENT_ID is set.
+//   --center     Optional. Defaults to ${FABRIC_CENTER_API_BASE} or
+//                http://localhost:7001/api.
+//   --identity-file  Optional path to fabric-identity.json (default
+//                ${PLEXUM_PROFILE_ROOT}/fabric-identity.json).
+//
+// Writes the (agent, api key, user id, email) tuple to identity file
+// after validating the key against Center via /auth/agent/login.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+	"time"
+
+	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/fabric"
+	"git.hangman-lab.top/hzhang/Plexum-fabric-channel-plugin/internal/identity"
+)
+
+func main() {
+	apiKey := flag.String("api-key", "", "Fabric Center API key (fak_...; required)")
+	agentID := flag.String("agent-id", "", "agent id (defaults to $AGENT_ID)")
+	centerBase := flag.String("center", "", "Center API base URL (default $FABRIC_CENTER_API_BASE or http://localhost:7001/api)")
+	identityFile := flag.String("identity-file", "", "identity registry path (default $PLEXUM_PROFILE_ROOT/fabric-identity.json)")
+	flag.Parse()
+
+	if *apiKey == "" {
+		fatalf("--api-key is required")
+	}
+	if *agentID == "" {
+		*agentID = os.Getenv("AGENT_ID")
+	}
+	if *agentID == "" {
+		fatalf("--agent-id required (or set AGENT_ID env)")
+	}
+	if *centerBase == "" {
+		*centerBase = os.Getenv("FABRIC_CENTER_API_BASE")
+	}
+	if *centerBase == "" {
+		*centerBase = "http://localhost:7001/api"
+	}
+	if *identityFile == "" {
+		*identityFile = identity.DefaultPath()
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	client := fabric.New(*centerBase)
+	sess, err := client.AgentLogin(ctx, *apiKey)
+	if err != nil {
+		fatalf("validate key against %s: %v", *centerBase, err)
+	}
+
+	reg, err := identity.Open(*identityFile)
+	if err != nil {
+		fatalf("open identity: %v", err)
+	}
+	reg.Set(*agentID, &identity.Entry{
+		FabricAPIKey: *apiKey,
+		FabricUserID: sess.User.ID,
+		FabricEmail:  sess.User.Email,
+		Enabled:      true,
+	})
+	if err := reg.Save(); err != nil {
+		fatalf("save identity: %v", err)
+	}
+
+	fmt.Printf("registered agent %s as fabric user %s (%s); %d guilds\n",
+		*agentID, sess.User.Email, sess.User.ID, len(sess.Guilds))
+	fmt.Printf("identity file: %s\n", *identityFile)
+	fmt.Println("restart the plexum gateway to pick up the new identity")
+}
+
+func fatalf(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "plexum-fabric-register: "+format+"\n", args...)
+	os.Exit(1)
+}