The per-session config written to the launch tmpdir embeds the mesh
keypair (secret key) but was created without a mode → 0644
(world/group-readable), which `claudemesh status` flags as
"perms 0644 — expected 0600". The enclosing mkdtemp dir is 0700, but
lock the file down too so the secret is never world-readable. File is
freshly created in a new tmpdir, so the mode applies on create.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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