russh only parses 4 PEM headers: OPENSSH, RSA, PKCS8, ENCRYPTED PKCS8.
EC keys (-----BEGIN EC PRIVATE KEY-----) with PKCS5 encryption silently
failed with "Could not read key".
Fix adds two fallbacks:
1. If russh can't parse the key, convert to PKCS8 via `openssl pkey`
which handles EC, DSA, and all other OpenSSL-supported formats
2. If the input doesn't start with -----BEGIN, try reading it as a
file path (supports ~ expansion) for keys stored on disk
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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