Add nightly workflow for testing with latest Triton

[Copilot]

Adds a GitHub Actions workflow that runs nightly to validate Iris against Triton's main branch, catching compatibility regressions early.

Workflow Design

• Trigger: Nightly at 2 AM UTC (cron: '0 2 * * *') + manual dispatch
• Image Build: Modifies apptainer/iris.def in-place to checkout Triton main instead of pinned commit
  • Uses pattern s/git checkout [0-9a-f]\{40\}/git checkout main/ for robustness
  • Stores image at ~/iris-apptainer-nightly/iris-dev-nightly.sif to avoid cache conflicts
• Test Matrix: 20 jobs covering 5 test suites × 4 GPU ranks (1, 2, 4, 8)
• Permissions: Minimal (contents: read)

Changes

• .github/workflows/iris-nightly-triton-test.yml - 201-line workflow
• README.md - Added workflow status badge

The workflow reuses existing test infrastructure (container_exec.sh, run_tests_distributed.py) and passes custom image via --image flag.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Nightly tests using latest Triton</issue_title>
<issue_description>We would like a GitHub action that runs nightly, builds a new apptainer image using the latest Triton version and run all the tests. Possible implementation is to create a new branch, in-place modify the .def file, then run the action but open to other implementations ideas. It is fine to not report anything and users can check the CI action logs to see success/failure or see CI result via email.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Nightly tests using latest Triton #365

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