Fix known-hosts trust deadlock, host key stat, and optional nozstd build

- stdout logger: release mutex while waiting on pause resume so prompt
  callbacks can log (fixes hang after trusting new hosts; known_hosts
  was written but Resume never ran).
- known hosts callback: stop borrowing the SSH dial throttle channel
  (could block or interact badly with parallel handshakes).
- host key path: use errors.Is(..., fs.ErrNotExist) for RootedPath.Stat
  wrapped errors; stat errors now fail fast instead of mis-read.
- public key path: same ErrNotExist check for authorized_keys miss.
- Build: optional DTAIL_NO_ZSTD=yes / nozstd tag for CGO-free builds;
  split zstd readers into tagged files.
- Docs/examples: firewalld note for port 2222, log prune timer+script,
  SSHBindAddress note, dserver unit disabled-by-default comment;
  firewalld helper script example.
- Regression test for stdout pause/mutex behavior.

Made-with: Cursor

1 files changed, 3 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 1ee56e5..2cf357d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -26,6 +26,9 @@ make clean
 # Enable ACL support (requires libacl-devel)
 DTAIL_USE_ACL=yes make build
 
+# Build without zstd (CGO-free cross-compiles; .zst logs unsupported)
+DTAIL_NO_ZSTD=yes make build
+
 # Enable proprietary features
 DTAIL_USE_PROPRIETARY=yes make build