feat(v0.4.1): warn when archiving workspace with active session

Archive now inspects .ctask/session.json before mutating task.yaml. A
fresh lease (heartbeat within 60s) triggers a warning. Interactive
stdin gets a y/N prompt (default N). Non-interactive stdin refuses
with a non-zero exit, which is safer than silently hiding an actively
writing workspace. Stale or missing leases pass through unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/archive.go

@@ -2,11 +2,13 @@ package cmd
 
 import (
 	"fmt"
+	"os"
 	"path/filepath"
 	"time"
 
 	"github.com/spf13/cobra"
 	"github.com/warrenronsiek/ctask/internal/config"
+	"github.com/warrenronsiek/ctask/internal/session"
 	"github.com/warrenronsiek/ctask/internal/workspace"
 )
 
@@ -26,6 +28,26 @@ func runArchive(cmd *cobra.Command, args []string) error {
 	roots := config.SearchRoots()
 	ws := resolveOne(roots, args[0], false)
 
+	// Active-session check: a fresh lease means a session is actively writing
+	// to this workspace. On TTY stdin we prompt. On non-TTY stdin we refuse —
+	// archiving under an active session silently is the most surprising
+	// failure mode, so the safe default is to exit non-zero.
+	if lease, err := session.ReadLease(session.LeasePath(ws.Path)); err == nil && lease != nil {
+		if session.IsFresh(lease, time.Now(), session.StaleLeaseAfter) {
+			fmt.Print(formatArchiveActiveWarning(lease, time.Now()))
+
+			if !isStdinTerminal() {
+				fmt.Println()
+				return fmt.Errorf("refusing to archive workspace with active session (stdin is not a terminal)")
+			}
+
+			if !session.ConfirmYN(os.Stdin, os.Stdout, "  Archive anyway? [y/N] ", false) {
+				fmt.Println("  Cancelled.")
+				return nil
+			}
+		}
+	}
+
 	now := time.Now().UTC().Truncate(time.Second)
 	ws.Meta.Status = "archived"
 	ws.Meta.ArchivedAt = &now
@@ -41,3 +63,33 @@ func runArchive(cmd *cobra.Command, args []string) error {
 
 	return nil
 }
+
+// formatArchiveActiveWarning renders the warning block shown when archive
+// detects a fresh lease.
+func formatArchiveActiveWarning(l *session.Lease, now time.Time) string {
+	startedAgo := now.Sub(l.StartedAt)
+	return fmt.Sprintf(
+		"[ctask] Warning: this workspace has an active session:\n"+
+			"  Host:     %s\n"+
+			"  Agent:    %s\n"+
+			"  Started:  %s (%s ago)\n\n"+
+			"  Archiving will hide it from list and resume.\n"+
+			"  The active session will continue but may behave unexpectedly.\n\n",
+		l.Hostname,
+		l.Agent,
+		l.StartedAt.Local().Format("2006-01-02 15:04"),
+		session.FormatAgo(startedAgo),
+	)
+}
+
+// isStdinTerminal reports whether os.Stdin is attached to a terminal.
+// Uses os.Stdin.Stat + ModeCharDevice to avoid adding a golang.org/x/term
+// dependency. Pipes and redirected files lack the ModeCharDevice bit on
+// every supported platform.
+func isStdinTerminal() bool {
+	fi, err := os.Stdin.Stat()
+	if err != nil {
+		return false
+	}
+	return (fi.Mode() & os.ModeCharDevice) != 0
+}