Migrate issue templates from Markdown to YAML form-based format

[knoan]

Description

The current issue templates (bug_report.md, feature_request.md) use Markdown format. YAML form-based templates (.yml) provide structured input fields, validation, and a guided form interface that improves data quality and consistency.

Current State

• bug_report.md — environment, steps to reproduce, expected/actual behaviour, workarounds, notes
• feature_request.md — challenge/use case, proposal, alternatives, notes

Tasks

• Convert bug_report.md to bug_report.yml with typed fields and required validation
• Convert feature_request.md to feature_request.yml with typed fields and required validation
• Add config.yml to configure template chooser — disable blank issues and add contact links to Discussions
• Remove old Markdown templates

Draft config.yml

blank_issues_enabled: false
contact_links:
  - name: Questions, Ideas, Tips & Tricks
    url: https://github.com/metreeca/blue/discussions
    about: Share questions, ideas, and tips in GitHub Discussions.

References

• Syntax for issue forms
• Syntax for GitHub's form schema
• Configuring issue templates