fix(plugins): chmod +x generated hook scripts + tolerate BrokenPipe in stdin write — closes ROADMAP #25 hotfix lane

Two bugs found in the plugin hook test harness that together caused
Linux CI to fail on 'hooks::tests::collects_and_runs_hooks_from_enabled_plugins'
with 'Broken pipe (os error 32)'. Three reproductions plus one rerun
failure on main today: 24120271422, 24120538408, 24121392171.

Root cause 1 (chmod, defense-in-depth): write_hook_plugin writes
pre.sh/post.sh/failure.sh via fs::write without setting the execute
bit. While the runtime hook runner invokes hooks via 'sh <path>' (so
the script file does not strictly need +x), missing exec perms can
cause subtle fork/exec races on Linux in edge cases.

Root cause 2 (the actual CI failure): output_with_stdin unconditionally
propagated write_all errors on the child's stdin pipe, including
BrokenPipe. A hook script that runs to completion in microseconds
(e.g. a one-line printf) can exit and close its stdin before the parent
finishes writing the JSON payload. Linux pipes surface this as EPIPE
immediately; macOS pipes happen to buffer the small payload, so the
race only shows on ubuntu CI runners. The parent's write_all raised
BrokenPipe, which output_with_stdin returned as Err, which run_command
classified as 'failed to start', making the test assertion fail.

Fix: (a) make_executable helper sets mode 0o755 via PermissionsExt on
each generated hook script, with a #[cfg(unix)] gate and a no-op
#[cfg(not(unix))] branch. (b) output_with_stdin now matches the
write_all result and swallows BrokenPipe specifically (the child still
ran; wait_with_output still captures stdout/stderr/exit code), while
propagating all other write errors. (c) New regression guard
generated_hook_scripts_are_executable under #[cfg(unix)] asserts each
generated .sh file has at least one execute bit set.

Surgical scope per gaebal-gajae's direction: chmod + pipe tolerance +
regression guard only. The deeper plugin-test sealing pass for ROADMAP
#25 + #27 stays in gaebal-gajae's OMX lane.

Verification:
- cargo test --release -p plugins → 35 passing, 0 failing
- cargo fmt -p plugins → clean
- cargo clippy -p plugins -- -D warnings → clean

Co-authored-by: gaebal-gajae <gaebal-gajae@layofflabs.com>

rust/crates/plugins/src/hooks.rs

@@ -337,7 +337,28 @@ impl CommandWithStdin {
         let mut child = self.command.spawn()?;
         if let Some(mut child_stdin) = child.stdin.take() {
             use std::io::Write as _;
-            child_stdin.write_all(stdin)?;
+            // Tolerate BrokenPipe: a hook script that runs to completion
+            // (or exits early without reading stdin) closes its stdin
+            // before the parent finishes writing the JSON payload, and
+            // the kernel raises EPIPE on the parent's write_all. That is
+            // not a hook failure — the child still exited cleanly and we
+            // still need to wait_with_output() to capture stdout/stderr
+            // and the real exit code. Other write errors (e.g. EIO,
+            // permission, OOM) still propagate.
+            //
+            // This was the root cause of the Linux CI flake on
+            // hooks::tests::collects_and_runs_hooks_from_enabled_plugins
+            // (ROADMAP #25, runs 24120271422 / 24120538408 / 24121392171
+            // / 24121776826): the test hook scripts run in microseconds
+            // and the parent's stdin write races against child exit.
+            // macOS pipes happen to buffer the small payload before the
+            // child exits; Linux pipes do not, so the race shows up
+            // deterministically on ubuntu runners.
+            match child_stdin.write_all(stdin) {
+                Ok(()) => {}
+                Err(error) if error.kind() == std::io::ErrorKind::BrokenPipe => {}
+                Err(error) => return Err(error),
+            }
         }
         child.wait_with_output()
     }
@@ -359,6 +380,18 @@ mod tests {
         std::env::temp_dir().join(format!("plugins-hook-runner-{label}-{nanos}"))
     }
 
+    fn make_executable(path: &Path) {
+        #[cfg(unix)]
+        {
+            use std::os::unix::fs::PermissionsExt;
+            let perms = fs::Permissions::from_mode(0o755);
+            fs::set_permissions(path, perms)
+                .unwrap_or_else(|e| panic!("chmod +x {}: {e}", path.display()));
+        }
+        #[cfg(not(unix))]
+        let _ = path;
+    }
+
     fn write_hook_plugin(
         root: &Path,
         name: &str,
@@ -368,21 +401,30 @@ mod tests {
     ) {
         fs::create_dir_all(root.join(".claude-plugin")).expect("manifest dir");
         fs::create_dir_all(root.join("hooks")).expect("hooks dir");
+
+        let pre_path = root.join("hooks").join("pre.sh");
         fs::write(
-            root.join("hooks").join("pre.sh"),
+            &pre_path,
             format!("#!/bin/sh\nprintf '%s\\n' '{pre_message}'\n"),
         )
         .expect("write pre hook");
+        make_executable(&pre_path);
+
+        let post_path = root.join("hooks").join("post.sh");
         fs::write(
-            root.join("hooks").join("post.sh"),
+            &post_path,
             format!("#!/bin/sh\nprintf '%s\\n' '{post_message}'\n"),
         )
         .expect("write post hook");
+        make_executable(&post_path);
+
+        let failure_path = root.join("hooks").join("failure.sh");
         fs::write(
-            root.join("hooks").join("failure.sh"),
+            &failure_path,
             format!("#!/bin/sh\nprintf '%s\\n' '{failure_message}'\n"),
         )
         .expect("write failure hook");
+        make_executable(&failure_path);
         fs::write(
             root.join(".claude-plugin").join("plugin.json"),
             format!(
@@ -496,4 +538,27 @@ mod tests {
             .iter()
             .any(|message| message == "later plugin hook"));
     }
+
+    #[test]
+    #[cfg(unix)]
+    fn generated_hook_scripts_are_executable() {
+        use std::os::unix::fs::PermissionsExt;
+
+        // given
+        let root = temp_dir("exec-guard");
+        write_hook_plugin(&root, "exec-check", "pre", "post", "fail");
+
+        // then
+        for script in ["pre.sh", "post.sh", "failure.sh"] {
+            let path = root.join("hooks").join(script);
+            let mode = fs::metadata(&path)
+                .unwrap_or_else(|e| panic!("{script} metadata: {e}"))
+                .permissions()
+                .mode();
+            assert!(
+                mode & 0o111 != 0,
+                "{script} must have at least one execute bit set, got mode {mode:#o}"
+            );
+        }
+    }
 }