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fix(login): allow storing credentials without OCI_REG_PLAIN_HTTP (fixes #769)

LoginOci gated ORAS's AllowPlaintextPut behind the plain-HTTP setting.
On a headless CI runner there is no credential helper, so the ORAS store
refuses to persist credentials at all ("putting plaintext credentials
is disabled") and `kpm registry login` cannot succeed against HTTPS-only
registries such as ghcr.io or docker.io — the only way to enable the
flag, OCI_REG_PLAIN_HTTP=on, also forces plain-HTTP transport.

AllowPlaintextPut is now enabled unconditionally, matching the Docker
and ORAS CLIs: in ORAS the plain-text config file is the last-resort
backend, server-specific helpers (credHelpers) and credsStore always
take precedence, so the flag only permits the fallback when no helper
is configured.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Peefy <xpf6677@163.com>