pixi: add root lint tasks

[rparolin]

What

The root pixi.toml declared ruff as a dependency but no task ever used it, so there was no repo-wide entry point for linting. Contributors had to know to invoke pre-commit directly, and AGENTS.md tells agents to prefer pixi tasks over raw tool invocations.

Adds a dedicated lint feature/environment with three tasks:

pixi run lint          # pre-commit run --all-files (the pre-commit.ci equivalent)
pixi run lint-staged   # pre-commit run (staged files only)
pixi run lint-install  # install the git hook

Why route through pre-commit instead of calling ruff directly

This was the interesting part. .pre-commit-config.yaml applies per-hook exclude patterns — generated *.pyi files and cuda/bindings/_internal/_fast_enum.py. A bare ruff check --fix . at the root does not honour those, and reformats 232 files that CI intentionally leaves alone (I hit this while developing the change). Routing every task through pre-commit keeps the task and pre-commit.ci in agreement by construction, with no second copy of the exclude list to drift.

ruff stays in the environment for ad-hoc use (pixi run -e lint ruff format <file>).

Notes for reviewers

• The feature uses no-default-feature = true, so the linters are not solved into the cu12/cu13/docs environments.
• Side effect: moving ruff out of [dependencies] leaves those three environments with an empty package set. They exist only to dispatch into the sub-package manifests, so this is intentional — but it does mean pixi run -e cu13 ruff ... no longer resolves. Happy to keep ruff in [dependencies] instead if you'd rather preserve that.
• pixi.lock migrates from format v6 to v7 as a side effect of re-locking with pixi 0.73. The other three workspace lockfiles are already on v7, so this brings the root in line.

Verification

pixi run lint runs the full suite green with zero file modifications — SPDX, generated-file seals, cuda-version pins, mypy, cython-lint, lychee, actionlint all pass.

---

PLC Local Security Evidence (Advisory)

Local advisory checks only. Authoritative release gates remain Pulse, SonarQube, Coverity, BlackDuck, nSpect, ScanSpect, OSRB, and Anchore — none of the rows below are release-gate results.

Check	Status	Tool	Details
Secrets	SKIP	Pulse Secret Scanner	Image pull denied — not authenticated to gitlab-master.nvidia.com:5005 (needs a PAT with read_registry)
SAST	PASS	Semgrep 1.157.0	0 high / 0 medium / 0 low over the changed files (bundled ruleset, severity filter ERROR)
License	SKIP	pip-licenses	pixi.lock is a conda+PyPI lockfile; pip-licenses reads only installed Python distribution metadata and has no pixi.lock parser. BlackDuck + OSRB remain authoritative
Container	N/A	—	No container files in this change
Security Review	PASS	inline	No auth, crypto, input-validation, or privilege-boundary code in this change

[rparolin]

Verified the empty-dispatcher-environment concern raised in the description: with ruff moved out of [dependencies], pixi run -e cu13 echo ok still activates cleanly and pixi run -e cu13 test-pathfinder dispatches into the sub-manifest and runs green (1095 passed, 4 skipped). The [activation.env] PIXI_ENVIRONMENT_NAME rewrite is unaffected by the environments having an empty package set.

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