Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/AGENTS.md

@@ -0,0 +1,113 @@
+# Agent Instructions for vLLM
+
+> These instructions apply to **all** AI-assisted contributions to `vllm-project/vllm`.
+> Breaching these guidelines can result in automatic banning.
+
+## 1. Contribution Policy (Mandatory)
+
+### Duplicate-work checks
+
+Before proposing a PR, run these checks:
+
+```bash
+gh issue view <issue_number> --repo vllm-project/vllm --comments
+gh pr list --repo vllm-project/vllm --state open --search "<issue_number> in:body"
+gh pr list --repo vllm-project/vllm --state open --search "<short area keywords>"
+```
+
+- If an open PR already addresses the same fix, do not open another.
+- If your approach is materially different, explain the difference in the issue.
+
+### No low-value busywork PRs
+
+Do not open one-off PRs for tiny edits (single typo, isolated style change, one mutable default, etc.). Mechanical cleanups are acceptable only when bundled with substantive work.
+
+### Accountability
+
+- Pure code-agent PRs are **not allowed**. A human submitter must understand and defend the change end-to-end.
+- The submitting human must review every changed line and run relevant tests.
+- PR descriptions for AI-assisted work **must** include:
+    - Why this is not duplicating an existing PR.
+    - Test commands run and results.
+    - Clear statement that AI assistance was used.
+
+### Fail-closed behavior
+
+If work is duplicate/trivial busywork, **do not proceed**. Return a short explanation of what is missing.
+
+---
+
+## 2. Development Workflow
+
+### Environment setup
+
+```bash
+# Install `uv` if you don't have it already:
+curl -LsSf https://astral.sh/uv/install.sh | sh
+
+# Always use `uv` for Python environment management:
+uv venv --python 3.12
+source .venv/bin/activate
+
+# Always make sure `pre-commit` and its hooks are installed:
+uv pip install -r requirements/lint.txt
+pre-commit install
+```
+
+### Installing dependencies
+
+```bash
+# If you are only making Python changes:
+VLLM_USE_PRECOMPILED=1 uv pip install -e .
+
+# If you are also making C/C++ changes:
+uv pip install -e .
+```
+
+### Running tests
+
+Tests require extra dependencies.
+All versions for test dependencies should be read from `requirements/test.txt`
+
+```bash
+# Install bare minimum test dependencies:
+uv pip install pytest pytest-asyncio tblib
+
+# Install additional test dependencies as needed, or install them all as follows:
+uv pip install -r requirements/test.txt
+
+# Run specific test from specific test file
+pytest tests/path/to/test.py -v -s -k test_name
+
+# Run all tests in directory
+pytest tests/path/to/dir -v -s
+```
+
+### Running linters
+
+```bash
+# Run all pre-commit hooks on staged files:
+pre-commit run
+
+# Run on all files:
+pre-commit run --all-files
+
+# Run a specific hook:
+pre-commit run ruff-check --all-files
+
+# Run mypy as it is in CI:
+pre-commit run mypy-3.10 --all-files --hook-stage manual
+```
+
+### Commit messages
+
+Add attribution using commit trailers such as `Co-authored-by:` (other projects use `Assisted-by:` or `Generated-by:`). For example:
+
+```text
+Your commit message here
+
+Co-authored-by: GitHub Copilot
+Co-authored-by: Claude
+Co-authored-by: gemini-code-assist
+Signed-off-by: Your Name <your.email@example.com>
+```