fix: fail when no secret GPG key is imported

[oiahoon]

Summary

• stop commit-signing setup immediately when an import produces no secret key
• remove the temporary key file before returning the new error

Why

GnuPG can successfully import public-only armored key data while --list-secret-keys returns no sec record. The existing setup then continues with an empty signing key ID and fails later without an actionable explanation.

This is the independent fail-fast follow-up discussed in #313.

Verification

• reproduced the public-only import with GnuPG 2.5.21; the updated function exits with the explicit error and removes private.key
• verified the valid path with a freshly generated ed25519 private key, a signed Git commit, and git verify-commit
• sh -n entrypoint.sh
• ShellCheck diagnostics match master with no new finding
• git diff --check

[andrii-bodnar]

@oiahoon thank you!