Add metric proposal issue template

[ebihara99999]

Purpose

Closes #42.

Metric additions need an issue-driven starting point because the metric definition, natural scope, and verification strategy need to be agreed before implementation.

Changes

• Add a GitHub issue form for new metric proposals.
• Require the proposal to state the metric name, motivation, metric source, natural scope, scope definition, value definition, verification strategy, and summary-model fit.
• Make the form distinguish RuboCop-backed metrics from custom metrics and point to the accepted scope naming and verification models from Draft: Renew CodeKeeper around AI workflow native MetricReport interface #40 and Add corpus-based verification for the metric report #41.
• Allow metrics that do not fit the current summary model to be proposed as summary-design discussions before implementation.
• Document in README that metric additions start from the new issue template.

Impact

No runtime behavior changes. This only changes contributor-facing GitHub and README documentation.

Verification

• ruby -e 'require "yaml"; data = YAML.safe_load_file(".github/ISSUE_TEMPLATE/new-metric.yml"); abort("missing body") unless data.fetch("body").is_a?(Array); puts "valid yaml"'
• bundle exec rubocop -A
• bundle exec rspec
• bundle exec rubocop
• git diff --check