docs(#100): add GitHub issue form

[wiktoriavh]

Summary

• Adds a single GitHub issue form (YAML) so new issues collect why, acceptance criteria, open questions, and out of scope.
• Optional bug fields (current/expected behavior, steps, environment) sit under a Bug details section.
• Disables blank issues (blank_issues_enabled: false). No default title prefix or labels.

This is the variant to try first. Mutually exclusive with the markdown template PR.

Closes #100

Test plan

• Merge (or otherwise land on default branch) so GitHub picks up templates
• Open New issue and confirm the form is the only option (no blank issue)
• Submit a feature-style issue skipping bug fields
• Submit a bug-style issue filling the bug fields
• Confirm the created issue body is readable markdown

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