feat: make llm-katan as default in docker compose up

[JaredforReal]

What this PR does / why we need it:
Adds a lightweight local OpenAI-compatible model (llm-katan) to the default Docker Compose stack for a fully self-contained out‑of‑the‑box experience.

Key Changes:

• New llm-katan service (Qwen/Qwen3-0.6B served as qwen3) with static IP 172.28.0.20:8002.
• Router config: set qwen3 as the default model.
• Added pre-download of Qwen3-0.6B to model bootstrap targets.
• New make target: docker-compose-up-core (stack without local model).
• Updated README with usage.
• Lint config: exclude models/ from markdown lint.

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 Root Directory

Owners: @rootfs, @Xunzhuo
Files changed:

• Dockerfile.extproc
• README.md

📁 config

Owners: @rootfs
Files changed:

• config/config.yaml

📁 deploy

Owners: @rootfs, @Xunzhuo
Files changed:

• deploy/docker-compose/docker-compose.yml

📁 tools

Owners: @yuluo-yx, @rootfs, @Xunzhuo
Files changed:

• tools/make/docker.mk
• tools/make/linter.mk
• tools/make/models.mk

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[rootfs]

lgtm, once the markdown link is fixed, it is good to go

cc @yossiovadia