Expand issue templates: feature request and blueprint proposal

[nojibe]

Problem

.github/ISSUE_TEMPLATE/ only contains bug_report.yml and config.yml. There is no structured path for feature requests or for proposing/contributing evaluation blueprints — a core contribution type for this project.

Proposed change

Add issue-form templates:

1. feature_request.yml — problem statement, proposed solution, alternatives, scope
2. blueprint_proposal.yml — domain/topic, why it matters, target models, draft points/rubric, links to sources

Review config.yml (blank_issues_enabled, contact links) so the chooser presents all templates cleanly.

Acceptance criteria

• Feature-request and blueprint-proposal templates appear in the "New issue" chooser
• Blueprint template links to docs/BLUEPRINT_FORMAT.md
• Labels auto-applied per template (e.g. enhancement, blueprint)