fix(ci): enforce legacy docs/ gate in Lint workflow

[zijiexia]

Summary

• The check-no-docs-changes pre-commit hook (introduced in 8750885) reads git diff --cached --name-only, which is empty on a fresh CI checkout. As a result pre-commit run --all-files in the Lint workflow has been a no-op for this gate, and many PRs landed docs/ changes since the hook was enabled ([diffusion] Generalize layerwise offload residency mixin to all components sgl-project/sglang#24593, Add IntraNode NVLink configration in PD disaggregation docs sgl-project/sglang#23329, [env] Make max KV chunk capacity configurable via SGLANG_MAX_KV_CHUNK_CAPACITY sgl-project/sglang#25120, feat: [2/2][DeepEP] Add waterfill load balancing for shared expert dispatch sgl-project/sglang#19290, [diffusion] Add performance mode defaults sgl-project/sglang#24491, ...).
• This adds a dedicated Reject changes under legacy docs/ step to .github/workflows/lint.yml that diffs origin/<base_ref>...HEAD and feeds the changed paths to the existing scripts/ci/check_no_docs_changes.py. The pre-commit hook and the script itself are unchanged — the script already accepts paths via sys.argv, so the allowlist (docs/conf.py, docs/_static/css/custom_log.css, docs/_static/js/deprecation_banner.js) is honored automatically.
• Bumps the checkout to fetch-depth: 0 so the merge base is available for the three-dot diff.

Why

Without a working CI gate, the rule was only enforced for contributors who had pre-commit installed; everyone else (or anyone using --no-verify) bypassed it silently.

Test plan

• Static: actionlint .github/workflows/lint.yml reports no new findings (one pre-existing setup-python@v4 warning unrelated to this PR).
• Local simulation against main with a probe commit touching docs/references/environment_variables.md → step exits 1 and prints the legacy-docs violation.
• Same simulation with docs/conf.py (allowlist) → exits 0.
• Same simulation with python/sglang/srt/__init__.py → exits 0.
• Real CI on this PR: the new step should fail on any PR touching non-allowlisted docs/ files, pass otherwise.

🤖 Generated with Claude Code

[zijiexia]

Superseded by upstream PR sgl-project#26322 — this PR was opened against my own fork by mistake.