skills: move 7 niche mlops/mcp skills to optional (#12474)
Built-in → optional-skills/:
mlops/training/peft → optional-skills/mlops/peft
mlops/training/pytorch-fsdp → optional-skills/mlops/pytorch-fsdp
mlops/models/clip → optional-skills/mlops/clip
mlops/models/stable-diffusion → optional-skills/mlops/stable-diffusion
mlops/models/whisper → optional-skills/mlops/whisper
mlops/cloud/modal → optional-skills/mlops/modal
mcp/mcporter → optional-skills/mcp/mcporter
Built-in mlops training kept: axolotl, trl-fine-tuning, unsloth.
Built-in mlops models kept: audiocraft, segment-anything.
Built-in mlops evaluation/research/huggingface-hub/inference all kept.
native-mcp stays built-in (documents the native MCP tool); mcporter was a
redundant alternative CLI.
Also: removed now-empty skills/mlops/cloud/ dir, refreshed
skills/mlops/models/DESCRIPTION.md and skills/mcp/DESCRIPTION.md to match
what's left, and synchronized both catalog pages (skills-catalog.md,
optional-skills-catalog.md).
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