feat(02-03): client socket, autostart daemon, switch handler, workspace wiring

- Client struct sends JSON requests to daemon over Unix socket
- EnsureDaemon auto-starts the daemon if not running (retry 50ms x 20)
- Switch handler uses FuzzyMatch + SwitchToWorkspace via i3 IPC
- InitWorkspaceResolver wires BuildTerminalWorkspaceMap + ResolveWorkspace
- sysattr_linux.go for Setsid detach on daemon spawn

client.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"net"
+	"os"
+	"os/exec"
+	"time"
+)
+
+// Client communicates with the vmux daemon over a Unix socket.
+type Client struct {
+	sockPath string
+}
+
+// NewClient creates a client targeting the given socket path.
+func NewClient(sockPath string) *Client {
+	return &Client{sockPath: sockPath}
+}
+
+// Send transmits a Request to the daemon and returns the Response.
+func (c *Client) Send(req Request) (*Response, error) {
+	conn, err := net.DialTimeout("unix", c.sockPath, 2*time.Second)
+	if err != nil {
+		return nil, fmt.Errorf("dial daemon: %w", err)
+	}
+	defer conn.Close()
+
+	conn.SetDeadline(time.Now().Add(5 * time.Second))
+
+	if err := json.NewEncoder(conn).Encode(req); err != nil {
+		return nil, fmt.Errorf("send request: %w", err)
+	}
+
+	var resp Response
+	if err := json.NewDecoder(conn).Decode(&resp); err != nil {
+		return nil, fmt.Errorf("read response: %w", err)
+	}
+
+	return &resp, nil
+}
+
+// EnsureDaemon checks if a daemon is already running on sockPath.
+// If not, it spawns one in the background and waits until it's reachable.
+func EnsureDaemon(sockPath string) error {
+	// Already running?
+	conn, err := net.DialTimeout("unix", sockPath, 200*time.Millisecond)
+	if err == nil {
+		conn.Close()
+		return nil
+	}
+
+	// Spawn daemon in background
+	exe, err := os.Executable()
+	if err != nil {
+		return fmt.Errorf("resolve executable: %w", err)
+	}
+
+	cmd := exec.Command(exe, "daemon")
+	cmd.SysProcAttr = newSysProcAttr()
+	cmd.Stdout = nil
+	cmd.Stderr = nil
+	cmd.Stdin = nil
+
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("start daemon: %w", err)
+	}
+
+	// Detach: don't wait for the child
+	go cmd.Wait()
+
+	// Retry until daemon is reachable (50ms x 20 = 1s max)
+	for i := 0; i < 20; i++ {
+		time.Sleep(50 * time.Millisecond)
+		conn, err := net.DialTimeout("unix", sockPath, 200*time.Millisecond)
+		if err == nil {
+			conn.Close()
+			return nil
+		}
+	}
+
+	return fmt.Errorf("daemon did not start within 1s")
+}