feat: support packing and pushing directly to a remote registry

[Hemanth-1354]

Description

Adds a --push flag to kit pack that streams the packed modelkit directly to
the remote registry specified by --tag, skipping local storage entirely.

Currently kit pack always writes layers/config/manifest to the local on-disk
cache before an optional separate kit push, meaning a model is stored twice
(source directory + local cache) before ever reaching the registry — costly in
CI environments where disk is limited. With --push, SaveModel and its layer
helpers write to a generic oras.Target instead of a concrete local repo, so
the same pack pipeline can target either local storage (default, unchanged
behavior) or a remote registry (remote.NewRepository) directly. Digests are
still computed via a temporary file (required to know the digest before
pushing), but nothing is persisted to the permanent local ModelKit cache when
--push is used.

Follows the approach agreed with @amisevsk in the issue thread (flag named
--push per that discussion).

Linked issues

Closes #1026

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[amisevsk]

Thanks @Hemanth-1354. I have a couple of minor comments, but otherwise this looks generally good. If you drop the changes to cli-reference.md and add the warning log on flags, this looks good to merge.

This doesn't fully hit what I was thinking for the issue, but is a great improvement nonetheless. A future (and significantly more complicated) improvement would be to do the push without storing the intermediate file on disk -- the tar writer could write directly into a remote connection and send the digest to seal the upload.

[Hemanth-1354]

Addressed all review feedback.

Thanks for the review @amisevsk.

1. Reverted docs/src/docs/cli/cli-reference.md, understood it's auto-generated at release time. Flags are already documented in Cobra help text so the generator picks them up.

2. Added warning in pkg/cmd/pack/cmd.go:177, when network flags (--plain-http, --tls-verify, --tls-cert, --cert, --key, --concurrency, --proxy) are explicitly passed without --push, a warning prints: "Warning: network-related flags are only applicable with --push flag". Uses cmd.Flags().Changed() so defaults don't trigger it.