Delete a batch of imports in one command, not one connection each

Replaces the connection throttle from the previous commit, which was a
workaround: it made a batch slower without changing that N sessions
meant N ssh connections, and the cap it picked was a guess at somebody
else's sshd config.

The batch now goes out as a single invocation. import::delete takes the
whole list, the remote script loops over the ids and prints one
`<id>\t<state>` line each, and the route settles every row from its own
line. So a batch of any size is one connection and cannot exceed
MaxStartups however many rows are selected -- and it is faster, since
it stopped paying a handshake per session.

Each id still reports on its own: deleted, missing, or failed, kept
apart because only "failed" is worth retrying. Ids the machine never
mentioned -- a connection that dropped part-way -- are reported as
unknown rather than defaulting to either answer, and a malformed id
fails only itself.

Verified end-to-end against a fake ssh that counts connections: a
9-session batch used one, every row settled, both copies of a session
recorded under two project directories went, and the id that was not
there failed with a message saying so rather than a connection error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/src/setups.rs

@@ -163,13 +163,6 @@ pub fn tidy(value: &str) -> Option<String> {
 /// Runs a launch to completion and returns its stdout.
 impl Transport {
     pub async fn capture(&self, launch: &Launch) -> Result<String> {
-        // See `CAPTURE_PERMITS`: caps how many of these run their ssh
-        // connection at once, so a batch doesn't open more than the remote
-        // sshd tolerates before it starts dropping them.
-        let _permit = super::session::transport::CAPTURE_PERMITS
-            .acquire()
-            .await
-            .expect("capture semaphore is never closed");
         let child = self.spawn(launch, super::session::transport::Streams::Piped)?;
         let output = child
             .wait_with_output()