fix(cli): call it a program, and say that stdin does not reach it

`tty7 run -- /no/such/binary` answered "no such shell on this machine".
The check in front of a spawn serves both the configured shell and a
command handed to `run`, and its three sentences all said "shell" —
so someone who typed `tty7 run -- ./build.sh` was told tty7 had
misunderstood what they asked for. They say "program" now, which is
true of both callers; the CLI already prefixes "spawning `…`" with the
context.

The other half is worse and is documented rather than changed:
`echo hi | tty7 run -- cat` never returns. The command reads the pane's
terminal, which nothing is typing into, so it waits for input that
cannot arrive — and there is no error, because an idle terminal is not
a failure. Output streams back; input does not go the other way.

Measured, after a first attempt sat for ten minutes: a bounded re-run
with `head -1` was still running at ten seconds with the pipe unread.
For a CLI whose stated audience is agents, `cmd | tty7 run -- …` is a
natural thing to try and an unbounded wait is the worst way to answer
it. Forwarding stdin would be a new capability; saying so is not, and
`sh -c 'cat < input.txt'` gets the input there today.

crates/tty7-cli/src/cli.rs

@@ -67,7 +67,14 @@ pub enum Command {
                       The environment comes from the server too, for the same reason, so \
                       `FOO=bar tty7 run -- ...` does not reach the command. Set it where \
                       the command can see it:\n\n    \
-                      tty7 run -- sh -c 'FOO=bar cargo test'"
+                      tty7 run -- sh -c 'FOO=bar cargo test'\n\n\
+                      Nor is your stdin forwarded: the command reads from the pane's \
+                      terminal, which nothing is typing into. Output is streamed back, \
+                      input does not go the other way, so `echo hi | tty7 run -- cat` \
+                      waits for input that cannot arrive — with no error, because as far \
+                      as the pane is concerned a terminal is simply idle. Give the \
+                      command its input directly:\n\n    \
+                      tty7 run -- sh -c 'cat < input.txt'"
     )]
     Run(RunArgs),
 

crates/tty7-core/src/daemon/pane.rs

@@ -122,10 +122,16 @@ struct SpawnConfig {
     shell: Option<ShellSpec>,
 }
 
-/// Why a configured shell cannot be run, in one sentence, before anything
-/// tries. Without it a missing program arrives at the window wrapped four
-/// deep — "daemon refused Spawn: spawn failed: Unable to spawn … (ENOENT: No
-/// such file or directory)" — and the one fact that matters is buried in it.
+/// Why a program cannot be run, in one sentence, before anything tries.
+/// Without it a missing program arrives at the window wrapped four deep —
+/// "daemon refused Spawn: spawn failed: Unable to spawn … (ENOENT: No such
+/// file or directory)" — and the one fact that matters is buried in it.
+///
+/// "Program", not "shell": the same check stands in front of the configured
+/// shell and of a command handed to `tty7 run`, and calling the latter a shell
+/// tells someone who typed `tty7 run -- ./build.sh` that tty7 misunderstood
+/// what they asked for. The caller supplies the context — the CLI prefixes
+/// "spawning `…`" — so the sentence only has to carry the fact.
 ///
 /// Only a program given as a path can be checked here; a bare name is resolved
 /// through PATH by the OS, and guessing at that would be worse than silence.
@@ -135,16 +141,16 @@ fn shell_program_problem(program: &str) -> Option<String> {
         return None;
     }
     let Ok(meta) = std::fs::metadata(path) else {
-        return Some(format!("no such shell on this machine: {program}"));
+        return Some(format!("no such program on this machine: {program}"));
     };
     if meta.is_dir() {
-        return Some(format!("the configured shell is a directory: {program}"));
+        return Some(format!("that program is a directory: {program}"));
     }
     #[cfg(unix)]
     {
         use std::os::unix::fs::PermissionsExt;
         if meta.permissions().mode() & 0o111 == 0 {
-            return Some(format!("the configured shell is not executable: {program}"));
+            return Some(format!("that program is not executable: {program}"));
         }
     }
     None
@@ -3191,7 +3197,7 @@ mod tests {
         let dir = tempfile::tempdir().expect("tempdir");
         let missing = dir.path().join("not-a-shell");
         let problem = shell_program_problem(&missing.to_string_lossy()).expect("a problem");
-        assert!(problem.contains("no such shell"), "{problem}");
+        assert!(problem.contains("no such program"), "{problem}");
         assert!(problem.contains("not-a-shell"), "{problem}");
         // One sentence: none of the layers this used to arrive wrapped in.
         assert!(!problem.contains("ENOENT"), "{problem}");