feat(v0.5.3): AttachExisting passive reattach helper

internal/session/attach.go

@@ -0,0 +1,29 @@
+package session
+
+import (
+	"github.com/warrenronsiek/ctask/internal/shell"
+)
+
+// attacher is the test seam used by AttachExisting. Production code calls
+// shell.AttachSession directly; tests override this variable to capture
+// invocations or simulate failures. Do not run tests that override this
+// variable in parallel — it is a package global.
+var attacher = shell.AttachSession
+
+// AttachExisting is the passive-reattach path. It is invoked when a tmux
+// session for the workspace already exists and the lease is fresh and local
+// (the original ctask owner is alive and heartbeating).
+//
+// AttachExisting performs no Preflight, writes no lease, captures no
+// manifest, starts no heartbeat, prints no banner, and runs no finalize.
+// It connects the user's terminal to the existing tmux session via
+// shell.AttachSession and returns when the user detaches or the session
+// ends. shell.AttachSession's contract handles failure-mode classification:
+// nil on clean exit, wrapped error on non-zero exit.
+//
+// If the session disappeared between the dispatcher's HasSession check
+// and this call, AttachSession returns an error and the user can retry —
+// the next invocation will hit the owner-create path.
+func AttachExisting(tmuxPath, name string) error {
+	return attacher(tmuxPath, name)
+}