restore, step 1: layout snapshots and dingger restore

the daemon saves the tmux layout when it changes: sessions, windows
(index, name, automatic-rename, layout, size, zoom, active), and panes
(directory, active, agent). a new history file for layout changes, a
rewrite for details, the last 50 kept, nothing saved while tmux is down.
when a different server runs (or none), the old server's last snapshot is
pinned as the restore candidate before the new server's snapshots start.

dingger restore rebuilds missing sessions by tmux id: panes split from the
newest so they keep their order, the saved layout applied at the saved
size, then the size released so windows follow clients again. a bare
session of the same name (a terminal's `new-session -As 0`) is replaced,
its clients switched over first; used sessions are kept. --dry-run,
--list, --from, --session.

e2e step 16 kills a server with a client attached like ghostty's, restarts
it, restores, and compares every window and pane; splitting the wrong
pane, not replacing bare sessions, not pinning, or keeping the saved size
each fail it. e2e keeps its state in its temp dir.

1 files changed, 1 insertions, 8 deletions

diff --git a/crates/dinggerd/src/hooks.rs b/crates/dinggerd/src/hooks.rs
index 524fb2d..46077cf 100644
--- a/crates/dinggerd/src/hooks.rs
+++ b/crates/dinggerd/src/hooks.rs
@@ -31,14 +31,7 @@ pub fn socket_path() -> PathBuf {
     if let Some(path) = std::env::var_os("DINGGER_AGENT_SOCKET") {
         return PathBuf::from(path);
     }
-    let state = std::env::var_os("DINGGER_STATE_DIR")
-        .map(PathBuf::from)
-        .or_else(|| {
-            std::env::var_os("HOME")
-                .map(|home| PathBuf::from(home).join(".local").join("state").join("dingger"))
-        })
-        .unwrap_or_else(|| PathBuf::from(".dingger"));
-    state.join("agent.sock")
+    crate::snapshot::state_dir().join("agent.sock")
 }
 
 #[derive(Debug, Default)]