Add GitHub contribution templates

[austin-smith]

What Changed

• Added structured bug report and feature request forms with the existing bug and enhancement labels.
• Added issue-template configuration that continues to allow blank issues.
• Added a pull request template covering scope, rationale, optional UI evidence, and validation.
• Updated AGENTS.md with branch, commit, pull request, and issue conventions.
• Corrected the testing guidance to document the existing Vitest suite.

Why

Crapdash had CI workflows but no standard issue or pull request intake. These templates establish consistent contribution expectations while tailoring environment details and sensitive-data guidance to a self-hosted dashboard.

Validation

• Issue-form YAML parsed successfully.
• pnpm lint
• pnpm test — 74 tests passed.
• pnpm build
• git diff --check