Add Prometheus client and scratchpad docs (#10)

- Add Prometheus HTTP client for querying Prometheus (Faraday)
- New file: `app/lib/r3x/client/prometheus.rb`
- Introduce `Result` Enumerable to parse series and expose metric/value/timestamp
- New file: `app/lib/r3x/client/prometheus/result.rb`
- Ignore `scratchpad/` and document its purpose in `AGENTS.md`

Author: zewelor

.gitignore

@@ -48,3 +48,4 @@
 
 /workflows
 /docs/mvp
+/scratchpad

AGENTS.md

@@ -159,6 +159,12 @@ This Rails app uses a small set of preferred libraries for common integration wo
 - Prefer duck typing: focus on behavior rather than names. Ask what the object can do and which methods it responds to; prefer ability over identity.
 - Reasoning: Duck typing is meritocratic. New classes can participate by exposing the right behavior without forcing central dispatch code to learn every subtype.
 
+## Scratchpad
+
+- `scratchpad/` is for quick test scripts, one-off experiments, and prototyping — anything outside the main implementation code and test suite.
+- The directory is gitignored. Do not place production code there.
+- Use it when you need to verify something quickly (e.g., hitting an API, testing a query) without writing a proper test.
+
 ## Scope
 
 - Apply these as defaults for new work.

app/lib/r3x/client/prometheus.rb

@@ -0,0 +1,23 @@
+module R3x
+  module Client
+    class Prometheus
+      def initialize(base_url:)
+        @connection = Faraday.new(url: base_url) do |f|
+          f.request :json
+          f.response :json
+        end
+      end
+
+      def query(promql)
+        response = connection.get("api/v1/query", query: promql)
+        raise "Prometheus query failed: #{response.status}" unless response.success?
+
+        Result.new(response.body["data"])
+      end
+
+      private
+
+      attr_reader :connection
+    end
+  end
+end

app/lib/r3x/client/prometheus/result.rb

@@ -0,0 +1,38 @@
+module R3x
+  module Client
+    class Prometheus
+      class Result
+        include Enumerable
+
+        attr_reader :result_type
+
+        def initialize(data)
+          @result_type = data["resultType"]
+          @series = data.fetch("result", []).map { |s| Series.new(s) }
+        end
+
+        def each(&block)
+          series.each(&block)
+        end
+
+        private
+
+        attr_reader :series
+
+        Series = Struct.new(:data) do
+          def metric
+            data["metric"]
+          end
+
+          def value
+            data.dig("value", 1)
+          end
+
+          def timestamp
+            data.dig("value", 0)
+          end
+        end
+      end
+    end
+  end
+end