Persist only named/prewarmed sessions, not implicit ones

Implicit sessions (dosh host with no --session) get a random
term-<millis>-<pid> name you can never reattach to, so persisting them just
left unreattachable holder processes lingering across restarts. The server
now only uses a persistent holder for prewarmed or explicitly-named
sessions; implicit ones use the in-process shell that dies on restart and
reaps on disconnect, as before. Adds a shared
protocol::{generate,is}_implicit_session_name as the single source of truth
for the name shape, used by both client (generate) and server (detect).

Server-only behavior change; wire VERSION unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bin/dosh-client.rs

@@ -37,7 +37,7 @@ use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::sync::Arc;
 use std::sync::atomic::{AtomicU64, Ordering};
-use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
+use std::time::{Duration, Instant};
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::{TcpListener, TcpStream, UdpSocket, UnixStream};
 use tokio::signal::unix::{SignalKind, signal};
@@ -1173,15 +1173,10 @@ fn select_session(requested: Option<&str>, _force_new: bool) -> String {
     if let Some(session) = requested {
         return session.to_string();
     }
-    unique_session_name()
-}
-
-fn unique_session_name() -> String {
-    let millis = SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .unwrap_or_default()
-        .as_millis();
-    format!("term-{millis}-{}", std::process::id())
+    // Implicit, ephemeral session: the server recognizes this name shape
+    // (`protocol::is_implicit_session_name`) and does NOT persist it across
+    // restarts, since you can never reattach to a random name.
+    protocol::generate_implicit_session_name()
 }
 
 fn ssh_bootstrap(