Add metric proposal issue template

.github/ISSUE_TEMPLATE/new-metric.yml

@@ -0,0 +1,144 @@
+name: New metric proposal
+description: Propose a metric addition before implementation.
+title: "Add metric: "
+labels:
+  - enhancement
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Metric additions start from an issue because the verification strategy is part of the design.
+        Complete the RuboCop-backed or custom-metric parts that match this proposal.
+        Use the accepted scope naming model from #40 and the verification model from #41.
+  - type: input
+    id: metric-name
+    attributes:
+      label: Metric name
+      description: Use the public metric key that would appear in `measurements[].metric`.
+      placeholder: "example: abc_metric"
+    validations:
+      required: true
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Motivation
+      description: Explain what review signal this metric adds and why it belongs in CodeKeeper.
+      placeholder: "What code quality risk should this metric make visible?"
+    validations:
+      required: true
+  - type: dropdown
+    id: metric-source
+    attributes:
+      label: Metric source
+      description: Select whether the value definition follows an existing RuboCop Metrics cop.
+      options:
+        - RuboCop-backed
+        - Custom
+    validations:
+      required: true
+  - type: dropdown
+    id: natural-scope
+    attributes:
+      label: Natural scope
+      description: Select the scope enumeration this metric should use.
+      options:
+        - Reuse method-like scopes
+        - Reuse class-like scopes
+        - Introduce a new scope type
+    validations:
+      required: true
+  - type: textarea
+    id: scope-definition
+    attributes:
+      label: Scope definition
+      description: |
+        For an existing enumeration, name the enumeration and any scope-specific exclusions.
+        For a new scope type, specify target AST nodes, exclusion rules, start and end line rules, and `scope_name` rules following #40.
+      placeholder: |
+        Existing scope example:
+        - Reuses method-like scopes.
+        - Excludes dynamic `define_method` names for the same reason as abc_metric and cyclomatic_complexity.
+
+        New scope type example:
+        - Target AST nodes:
+        - Exclusion rules:
+        - Start/end line rules:
+        - `scope_name` rules, including namespace and static/dynamic `self` behavior:
+    validations:
+      required: true
+  - type: textarea
+    id: value-definition
+    attributes:
+      label: Value definition
+      description: |
+        For RuboCop-backed metrics, name the RuboCop cop and the exact calculation behavior to adopt.
+        For custom metrics, provide a precise self-contained definition.
+      placeholder: |
+        RuboCop-backed:
+        - Cop:
+        - Configuration or calculator behavior:
+        - Any intentional difference from RuboCop:
+
+        Custom:
+        - Definition:
+        - What increments or changes the value:
+        - What does not count:
+    validations:
+      required: true
+  - type: textarea
+    id: verification-strategy
+    attributes:
+      label: Verification strategy
+      description: |
+        RuboCop-backed metrics must extend the corpus differential comparison from #41.
+        Custom metrics must include a definition document, metamorphic properties, and golden snapshots because no external oracle exists.
+      placeholder: |
+        RuboCop-backed:
+        - Differential comparison changes:
+        - Corpus coverage to add:
+        - Accepted differences, if any:
+
+        Custom:
+        - Definition document location:
+        - Metamorphic properties:
+        - Golden snapshot coverage:
+    validations:
+      required: true
+  - type: dropdown
+    id: summary-model-fit
+    attributes:
+      label: Summary-model fit
+      description: |
+        CodeKeeper summary hotspots sort measurements by descending numeric value.
+        A metric that does not satisfy this needs a summary design discussion before implementation.
+      options:
+        - Fits the current summary model
+        - Needs summary design discussion before implementation
+    validations:
+      required: true
+  - type: textarea
+    id: summary-model-details
+    attributes:
+      label: Summary-model details
+      description: |
+        If the metric fits the current summary model, confirm that the value is numeric, larger values mean more review attention, and descending `top_hotspots` is meaningful.
+        If it does not fit, describe the summary behavior that needs design discussion before implementation.
+      placeholder: |
+        Fits current model:
+        - Value is numeric:
+        - Larger values mean more review attention:
+        - Descending `top_hotspots` is meaningful:
+
+        Needs design discussion:
+        - Why the current summary model does not fit:
+        - Proposed summary behavior:
+    validations:
+      required: true
+  - type: textarea
+    id: open-questions
+    attributes:
+      label: Open questions
+      description: List unresolved design or verification questions, or write `None`.
+      placeholder: "None"
+    validations:
+      required: true

README.md

@@ -140,6 +140,12 @@ To install this gem onto your local machine, run `bundle exec rake install`. To
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/ebihara99999/code_keeper. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [code of conduct](https://github.com/ebihara99999/code_keeper/blob/master/CODE_OF_CONDUCT.md).
 
+### Adding a metric
+
+Metric additions start from a GitHub issue using the new metric proposal template. Open the issue before implementation so the metric name, natural scope, value definition, verification strategy, and summary-model fit can be agreed first.
+
+For RuboCop-backed metrics, the issue should describe how the corpus differential check will be extended. For custom metrics, the issue should define the metric precisely and describe the metamorphic properties and golden snapshots that will verify it.
+
 ## License
 
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).