fix(sling): clear BEADS_DIR env var when creating auto-convoys

When running from a crew workspace, BEADS_DIR is set to the rig's beads
directory. This caused auto-convoy creation to fail because bd would use
the rig's database (prefix=bd) instead of discovering the HQ database
(prefix=hq) from the working directory.

The fix clears BEADS_DIR from the environment when running bd commands
for convoy creation, allowing bd to discover the correct database from
the townBeads directory.

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

internal/cmd/sling_convoy.go

@@ -56,6 +56,26 @@ func isTrackedByConvoy(beadID string) string {
 	return convoyID
 }
 
+// filterEnvExcluding returns a copy of the environment with specified keys removed.
+// This is used to prevent rig-specific environment variables from affecting
+// commands that need to run in a different context (e.g., HQ beads).
+func filterEnvExcluding(env []string, excludeKeys ...string) []string {
+	result := make([]string, 0, len(env))
+	for _, e := range env {
+		exclude := false
+		for _, key := range excludeKeys {
+			if strings.HasPrefix(e, key+"=") {
+				exclude = true
+				break
+			}
+		}
+		if !exclude {
+			result = append(result, e)
+		}
+	}
+	return result
+}
+
 // createAutoConvoy creates an auto-convoy for a single issue and tracks it.
 // Returns the created convoy ID.
 func createAutoConvoy(beadID, beadTitle string) (string, error) {
@@ -85,8 +105,13 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 		createArgs = append(createArgs, "--force")
 	}
 
+	// Clear BEADS_DIR so bd discovers the database from townBeads directory
+	// instead of using the rig's beads (which has a different prefix)
+	cleanEnv := filterEnvExcluding(os.Environ(), "BEADS_DIR")
+
 	createCmd := exec.Command("bd", append([]string{"--no-daemon"}, createArgs...)...)
 	createCmd.Dir = townBeads
+	createCmd.Env = cleanEnv
 	createCmd.Stderr = os.Stderr
 
 	if err := createCmd.Run(); err != nil {
@@ -98,6 +123,7 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 	depArgs := []string{"--no-daemon", "dep", "add", convoyID, trackBeadID, "--type=tracks"}
 	depCmd := exec.Command("bd", depArgs...)
 	depCmd.Dir = townBeads
+	depCmd.Env = cleanEnv
 	depCmd.Stderr = os.Stderr
 
 	if err := depCmd.Run(); err != nil {