docs: add structured bug report issue template

[ryanpetrello]

What

Adds .github/ISSUE_TEMPLATE/bug_report.yml — a structured bug report form with two required sections:

• Environment — prefilled with Fromager version, Python version, OS/arch, container/local
• Bug description — prefilled prompts for expected behavior, actual behavior, and logs

Auto-applies the bug label and sets a [Bug]: title prefix.

Why

Closes #967 — freeform bug reports often miss critical details needed to reproduce failures.

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds a structured GitHub issue template at .github/ISSUE_TEMPLATE/bug_report.yml for bug reporting in the fromager project. The template defines required fields for package details, build stage, fromager version, command run, and expected/actual behavior, along with optional fields for error output, package settings, constraints, and environment information. The template auto-applies the bug label to created issues and uses appropriate renderers (shell for logs, YAML for settings).

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding a structured bug report issue template to the documentation.
Linked Issues check	✅ Passed	The PR fully implements all required fields and features from issue #967: all 11 fields present, build-step dropdown with correct options, auto-applied bug label, proper render types (shell/yaml), and environment prefill with required variables.
Out of Scope Changes check	✅ Passed	The PR contains only the single file addition (.github/ISSUE_TEMPLATE/bug_report.yml) directly required by issue #967; no extraneous changes detected.
Description check	✅ Passed	The pull request description clearly explains what (a bug report template) and why (to collect critical details for reproducible bug reports, closing #967).

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[ryanpetrello]

For reviewers, here's a preview of the form:

https://github.com/ryanpetrello/fromager/issues/new?template=bug_report.yml