add sshd.sandbox_dir config option (#1622)

* add sshd.sandbox_dir config option

Sanitize SSH profile paths (ssh.go:514,683,719) — restrict os.Create(a[0]) to a safe directory.
Add a config option in the config file to specify the sandbox directory. For backwards compatibility, if the config is not specified, keep the current behavior.

* update default and example

* use os.TempDir() for sshd.sandbox_dir default

* split sandbox path validation into separate conditionals

Separate the combined && check in sshSanitizeFilePath into two distinct
conditionals with specific error messages: one for paths resolving to the
sandbox directory itself, and one for paths outside the sandbox.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

* fix: trim leading zeros from p256 signature swap result

bigmod.Nat.Bytes() returns fixed-size 32-byte slices, but ASN.1 INTEGER
parsing strips leading zeros. This caused a flaky test failure (~1/256
chance) when the S value's high byte was zero.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

---------

Co-authored-by: Claude <svc-devxp-claude@slack-corp.com>

examples/config.yml

@@ -204,6 +204,12 @@ punchy:
   # Trusted SSH CA public keys. These are the public keys of the CAs that are allowed to sign SSH keys for access.
   #trusted_cas:
     #- "ssh public key string"
+  # sandbox_dir restricts file paths for profiling commands (start-cpu-profile, save-heap-profile,
+  # save-mutex-profile) to the specified directory. Relative paths will be resolved within this directory,
+  # and absolute paths outside of it will be rejected. Default is $TMP/nebula-debug.
+  # The directory is NOT automatically created.
+  # Overriding this to "" is the same as "/" and will allow overwriting any path on the host.
+  #sandbox_dir: /var/tmp/nebula-debug
 
 # EXPERIMENTAL: relay support for networks that can't establish direct connections.
 relay: