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, 27 insertions, 0 deletions

diff --git a/crates/dinggerd/src/main.rs b/crates/dinggerd/src/main.rs
index 3707d51..2129bd2 100644
--- a/crates/dinggerd/src/main.rs
+++ b/crates/dinggerd/src/main.rs
@@ -5,6 +5,8 @@ mod integrations;
 mod manifest_update;
 mod names;
 mod notify;
+mod restore;
+mod snapshot;
 mod tmux;
 mod watcher;
 
@@ -52,6 +54,24 @@ enum Command {
     },
     /// Wake the watcher so focus changes show up immediately (tmux hooks).
     Poke,
+    /// Restore tmux sessions saved before tmux last stopped (a crash,
+    /// shutdown, or kill-server): windows, panes, layouts, directories.
+    /// Only adds what is missing; a bare session of the same name (what a
+    /// terminal creates on startup) is replaced, its clients moved over.
+    Restore {
+        /// Show what would happen without changing anything.
+        #[arg(long)]
+        dry_run: bool,
+        /// List the restore candidate and saved snapshots.
+        #[arg(long)]
+        list: bool,
+        /// Restore from this snapshot file instead of the candidate.
+        #[arg(long)]
+        from: Option<String>,
+        /// Restore only these sessions.
+        #[arg(long = "session")]
+        sessions: Vec<String>,
+    },
     /// Install agent integrations (hook extensions that report state).
     Install {
         /// Agent to install the integration for.
@@ -112,6 +132,13 @@ fn main() {
             // Best effort: a stopped watcher just means nothing to wake.
             let _ = hooks::poke();
         }
+        Command::Restore { dry_run, list, from, sessions } => {
+            let result = if list { restore::list() } else { restore::run(from.as_deref(), &sessions, dry_run) };
+            if let Err(err) = result {
+                eprintln!("dingger: {err}");
+                std::process::exit(1);
+            }
+        }
         Command::Install { agent } => {
             if let Err(err) = integrations::install(&agent) {
                 eprintln!("dingger: {err}");