feat: SessionLifecycleManager — auto-detect and repair dead tmux sessions

- Add internal/lifecycle/manager.go with Manager struct, Run() ticker loop
  (15s interval), EnsureAllSessions() for boot-time session creation, and
  reconcile() that recreates idle sessions and recovers working ones via
  SetFailed + CreateSession
- Add state.SetFailed() to record crash timestamp on SessionState
- Add internal/lifecycle/manager_test.go with mock tmux client and 3 tests:
  TestReconcileCreatesDeadSession, TestReconcileRecoversCrashedSession,
  TestEnsureAllSessions — all pass
- Wire lifecycle.Manager into cmd/claude-failover/main.go after state init

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/tmux/client.go

@@ -0,0 +1,89 @@
+// Package tmux provides an interface and implementation for controlling tmux sessions.
+package tmux
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// Client defines the operations the daemon needs on tmux sessions.
+type Client interface {
+	// HasSession returns true if the named session exists.
+	HasSession(name string) bool
+	// CreateSession creates a new detached session with an optional working directory.
+	CreateSession(name, workdir string) error
+	// KillSession destroys the named session.
+	KillSession(name string) error
+	// SendKeys sends key strokes to the first window of a session.
+	SendKeys(session, keys string) error
+	// CapturePaneTail returns the last n lines from the session's active pane.
+	CapturePaneTail(session string, lines int) (string, error)
+}
+
+// ExecClient implements Client by shelling out to the tmux binary.
+type ExecClient struct{}
+
+// NewExecClient returns a ready-to-use ExecClient.
+func NewExecClient() *ExecClient {
+	return &ExecClient{}
+}
+
+// run executes a tmux subcommand and returns combined output.
+func run(args ...string) (string, error) {
+	cmd := exec.Command("tmux", args...)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &out
+	err := cmd.Run()
+	return strings.TrimSpace(out.String()), err
+}
+
+// HasSession returns true when `tmux has-session` exits 0.
+func (c *ExecClient) HasSession(name string) bool {
+	_, err := run("has-session", "-t", name)
+	return err == nil
+}
+
+// CreateSession creates a new detached tmux session.
+// If workdir is non-empty, the session starts in that directory.
+func (c *ExecClient) CreateSession(name, workdir string) error {
+	args := []string{"new-session", "-d", "-s", name}
+	if workdir != "" {
+		args = append(args, "-c", workdir)
+	}
+	out, err := run(args...)
+	if err != nil {
+		return fmt.Errorf("tmux new-session %q: %w (%s)", name, err, out)
+	}
+	return nil
+}
+
+// KillSession destroys the named session.
+func (c *ExecClient) KillSession(name string) error {
+	out, err := run("kill-session", "-t", name)
+	if err != nil {
+		return fmt.Errorf("tmux kill-session %q: %w (%s)", name, err, out)
+	}
+	return nil
+}
+
+// SendKeys sends key strokes to the first pane of the session.
+func (c *ExecClient) SendKeys(session, keys string) error {
+	out, err := run("send-keys", "-t", session, keys, "Enter")
+	if err != nil {
+		return fmt.Errorf("tmux send-keys %q: %w (%s)", session, err, out)
+	}
+	return nil
+}
+
+// CapturePaneTail captures the last n lines from the session's active pane.
+func (c *ExecClient) CapturePaneTail(session string, lines int) (string, error) {
+	out, err := run("capture-pane", "-p", "-t", session,
+		"-S", fmt.Sprintf("-%d", lines))
+	if err != nil {
+		return "", fmt.Errorf("tmux capture-pane %q: %w (%s)", session, err, out)
+	}
+	return out, nil
+}