Deprecate train command with warning before future removal #8197

Job	Run time
Detect changes	5s
Build coglet wheel	1m 43s
Lint Go	39s
Compute test shards	4s
Build cog CLI	37s
Test Go (macos-latest)	41s
Check LLM docs	32s
Test Go (ubuntu-latest)	27s
Format Go	52s
Fuzz Go	2m 22s
Format Python	-1s
Validate coglet version	-1s
Format Rust	-1s
Lint Rust	-1s
Lint Rust (deny)	-1s
Build SDK	38s
Test Rust	-1s
Test integration (shard 0)	1s
Test integration (shard 3)	1s
Test integration (shard 2)	1s
Test integration (shard 1)	1s
Test coglet-python (${{ matrix.python-version }})	0s
Test Python ${{ matrix.python-version }}	0s
Lint Python	0s
CI Complete	3s
Release Dry Run	1s
	8m 42s

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