feat: add town-level activity signal for Gas Town daemon (bd-v8ku)

Writes activity.json to ~/gt/daemon/ when bd runs inside a Gas Town
workspace. This enables the daemon to detect bd usage and adjust its
polling frequency with exponential backoff.

Best-effort: silently skips if not in Gas Town or on any error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"os"
 	"os/signal"
@@ -312,6 +313,9 @@ var rootCmd = &cobra.Command{
 		// Set up signal-aware context for graceful cancellation
 		rootCtx, rootCancel = signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 
+		// Signal Gas Town daemon about bd activity (best-effort, for exponential backoff)
+		defer signalGasTownActivity()
+
 		// Apply verbosity flags early (before any output)
 		debug.SetVerbose(verboseFlag)
 		debug.SetQuiet(quietFlag)
@@ -944,6 +948,80 @@ var rootCmd = &cobra.Command{
 // Configurable via config file or BEADS_FLUSH_DEBOUNCE env var (e.g., "500ms", "10s")
 // Defaults to 5 seconds if not set or invalid
 
+// signalGasTownActivity writes an activity signal for Gas Town daemon.
+// This enables exponential backoff based on bd usage detection (gt-ws8ol).
+// Best-effort: silent on any failure, never affects bd operation.
+func signalGasTownActivity() {
+	// Determine town root
+	// Priority: GT_ROOT env > detect from cwd path > skip
+	townRoot := os.Getenv("GT_ROOT")
+	if townRoot == "" {
+		// Try to detect from cwd - if under ~/gt/, use that as town root
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return
+		}
+		gtRoot := filepath.Join(home, "gt")
+		cwd, err := os.Getwd()
+		if err != nil {
+			return
+		}
+		if strings.HasPrefix(cwd, gtRoot+string(os.PathSeparator)) {
+			townRoot = gtRoot
+		}
+	}
+
+	if townRoot == "" {
+		return // Not in Gas Town, skip
+	}
+
+	// Ensure daemon directory exists
+	daemonDir := filepath.Join(townRoot, "daemon")
+	if err := os.MkdirAll(daemonDir, 0755); err != nil {
+		return
+	}
+
+	// Build command line from os.Args
+	cmdLine := strings.Join(os.Args, " ")
+
+	// Determine actor (use package-level var if set, else fall back to env)
+	actorName := actor
+	if actorName == "" {
+		if bdActor := os.Getenv("BD_ACTOR"); bdActor != "" {
+			actorName = bdActor
+		} else if user := os.Getenv("USER"); user != "" {
+			actorName = user
+		} else {
+			actorName = "unknown"
+		}
+	}
+
+	// Build activity signal
+	activity := struct {
+		LastCommand string `json:"last_command"`
+		Actor       string `json:"actor"`
+		Timestamp   string `json:"timestamp"`
+	}{
+		LastCommand: cmdLine,
+		Actor:       actorName,
+		Timestamp:   time.Now().UTC().Format(time.RFC3339),
+	}
+
+	data, err := json.Marshal(activity)
+	if err != nil {
+		return
+	}
+
+	// Write atomically (write to temp, rename)
+	activityPath := filepath.Join(daemonDir, "activity.json")
+	tmpPath := activityPath + ".tmp"
+	// nolint:gosec // G306: 0644 is appropriate for a status file
+	if err := os.WriteFile(tmpPath, data, 0644); err != nil {
+		return
+	}
+	_ = os.Rename(tmpPath, activityPath)
+}
+
 func main() {
 	if err := rootCmd.Execute(); err != nil {
 		os.Exit(1)