CI: replace micromamba + environment.yaml with pixi

[lmoresi]

Summary

Replaces the failing micromamba + `environment.yaml` install step with `prefix-dev/setup-pixi` against the committed `pixi.lock`. The CI now uses the same pinned dependency set developers use locally, deterministically, in seconds rather than 60+ minutes.

Why

Since 2026-04-30 every push to `development` and every PR has failed CI with the runner timing out:

```text
Resolving Environment ⧖ Starting # 02:51:02
##[error]The runner has received a shutdown signal. # 03:57:25
```

Three consecutive `push`-event runs on `development` have failed the same way; PRs #154, #163 inherited it. The 66-minute window is entirely in micromamba's conda-forge dependency resolve — tests never start. Diagnosis chain: `environment.yaml` has loose constraints (`python <= 3.11`, an exact pin on `petsc=3.21.5` that's now a year-old in conda-forge, and several unpinned packages including `pykdtree` which UW3 doesn't even use any more), and recent conda-forge package state shifts have put the solver into deep backtracking.

What changes

Step	Before	After
Install	`mamba-org/setup-micromamba@v2` + `environment.yaml` (60+ min, often timing out)	`prefix-dev/setup-pixi@v0.9.4` + `pixi.lock` (`frozen: true`), seconds
Build	`pip install -e .` (editable, against project policy per CLAUDE.md)	`pixi run -e dev build` (= `pip install . --no-build-isolation`, non-editable)
Test	`./scripts/test.sh --p 2`	`pixi run -e dev ./scripts/test.sh --p 2`
Timeout	none (runner default ≳ 6h)	explicit 60 min cap

What does NOT change

• `environment.yaml` is kept for now because PR Add Gadi Singularity container build files for UW3 #133 (Gadi Singularity container) consumes it; removing it across the boundary of an open PR would create a cross-PR ordering hazard. We can prune it after Add Gadi Singularity container build files for UW3 #133 lands or after a deliberate cleanup pass.
• The test invocation, suite, and parallel-rank count are identical.

Test plan

This PR's own CI run is the test plan — if it completes green in <10 min instead of timing out at ~90 min, we've fixed the unblocker.

If the pixi step succeeds but tests then expose a real divergence vs the previous environment (different Python minor, different package versions), that's a useful signal — the existing `environment.yaml` has been silently falling out of sync with `pixi.lock` for a while, and tests passing under one but not the other tells us where the latent bugs are.

Underworld development team with AI support from Claude Code

[Copilot]

Pull request overview

This PR updates the primary GitHub Actions CI workflow to use Pixi + the committed pixi.lock for dependency installation, replacing the previously timing-out micromamba + environment.yaml solve step. This aligns CI with the repo’s pinned developer environment and should eliminate long conda-forge backtracking on runners.

Changes:

• Replace micromamba/environment.yaml setup with prefix-dev/setup-pixi using the dev Pixi environment and frozen: true.
• Run build and tests via pixi run -e dev ... to ensure all commands execute inside the Pixi environment.
• Add an explicit job timeout (timeout-minutes: 60) to prevent unbounded CI hangs.

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