Add a Fuzzlyn triage skill

[jakobbotsch]

This skill triages Fuzzlyn weekend runs. It:

• Downloads the artifacts and selects failures found by Fuzzlyn. Currently it focuses only on assertion failures.
• Downloads the Core_Root that Fuzzlyn was testing.
• Uses superpmi to find a context that reproduces the failure and mcs to extract that context.
• Uses superpmi to create a jitdump of the failure.
• Analyses the jitdump and JIT source to give its own input on what the issue may be.
• Creates a zip with the repro.mc, jitdump.txt, and an issue.md file that can be used to open a github issue

#127745 and #127747 are examples of what the issue.md file looks like once posted. The idea is to give enough detail that we can evaluate whether it sounds right or not, possibly come up with our own revisions, and then have CCA pick it up and actually fix the issue.

[Copilot]

Pull request overview

This PR adds a new GitHub Copilot skill definition (fuzzlyn-triage) intended to guide initial triage of Fuzzlyn weekend run failures by downloading run artifacts, isolating a reproducing SuperPMI context, producing a jitdump, and packaging artifacts plus an issue template for later filing.

Changes:

• Added a new skill markdown (SKILL.md) describing an end-to-end triage workflow for Fuzzlyn “Issues” artifacts.
• Documented how to extract a single failing SuperPMI context with mcs and generate a jitdump with SuperPMI JIT options.
• Documented the expected contents/structure of the generated details.zip and issue.md.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

[jakobbotsch]

PTAL @dotnet/jit-contrib