feat(v0.4): add session Lease type with JSON round-trip

internal/session/lease.go

@@ -0,0 +1,91 @@
+package session
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/user"
+	"path/filepath"
+	"time"
+)
+
+// Lease is the on-disk format of .ctask/session.json. It identifies the ctask
+// process holding this workspace open and records a heartbeat timestamp so
+// that crashed sessions can be detected and cleaned up by a later session.
+type Lease struct {
+	SessionID       string    `json:"session_id"`
+	PID             int       `json:"pid"`
+	Hostname        string    `json:"hostname"`
+	Username        string    `json:"username"`
+	Agent           string    `json:"agent"`
+	Mode            string    `json:"mode"`
+	StartedAt       time.Time `json:"started_at"`
+	LastHeartbeatAt time.Time `json:"last_heartbeat_at"`
+	Terminal        string    `json:"terminal"`
+}
+
+// LeasePath returns the absolute path of the lease file for the given workspace.
+func LeasePath(wsDir string) string {
+	return filepath.Join(wsDir, ".ctask", "session.json")
+}
+
+// WriteLease marshals a lease to JSON and writes it to path.
+// The .ctask/ directory is created if needed.
+func WriteLease(path string, l *Lease) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return err
+	}
+	data, err := json.MarshalIndent(l, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(path, data, 0644)
+}
+
+// ReadLease reads and parses a lease file. A missing file returns the
+// stdlib os.ErrNotExist (via os.ReadFile), which callers should detect
+// with errors.Is.
+func ReadLease(path string) (*Lease, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	var l Lease
+	if err := json.Unmarshal(data, &l); err != nil {
+		return nil, fmt.Errorf("parsing lease: %w", err)
+	}
+	return &l, nil
+}
+
+// currentUsername returns the current OS username, falling back to "unknown"
+// on error. Never panics.
+func currentUsername() string {
+	u, err := user.Current()
+	if err != nil || u == nil {
+		return "unknown"
+	}
+	if u.Username == "" {
+		return "unknown"
+	}
+	return u.Username
+}
+
+// currentHostname returns the current hostname, falling back to "unknown".
+func currentHostname() string {
+	h, err := os.Hostname()
+	if err != nil || h == "" {
+		return "unknown"
+	}
+	return h
+}
+
+// currentTerminal is a best-effort terminal identifier based on common env vars.
+// Returns "unknown" if none are set.
+func currentTerminal() string {
+	for _, k := range []string{"TERM_PROGRAM", "WT_SESSION", "TERM"} {
+		if v := os.Getenv(k); v != "" {
+			return v
+		}
+	}
+	return "unknown"
+}