Add Mermaid diagrams

Author: Michael Hallik

docs/images/CONTRIBUTING-diagram.md

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+```mermaid
+flowchart TD
+  A[Fork repo] --> B[Create feature branch]
+  B --> C[Code, test & commit]
+  C --> D[Push to fork]
+  D --> E[Open pull request to main]
+  E --> F[GitHub Actions run]
+
+  F -->|✅ Pass| G[Code review]
+  F -->|❌ Fail| L[Push changes to branch]
+  L --> F
+
+  G -->|✅ Approved| H[PR merged into main]
+  G -->|❌ Changes requested| L
+```

docs/images/github_actions.md

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+# test.yml
+
+```mermaid
+flowchart TD
+  A[Trigger: push or PR to main] --> B[Job: test - ubuntu-latest]
+  B --> C
+
+  subgraph StepGroup [ ]
+    direction TB
+    C[Checkout Repository]
+    C --> D[Set Up Python]
+    D --> E[Cache Poetry Dependencies]
+    E --> F[Install Dependencies]
+    F --> G[Show Poetry Configuration]
+    G --> H[Add Poetry 'bin' Dir To PATH]
+    H --> I[Log sys.path]
+    I --> J[Run Unit Tests]
+    J --> K[Install pytest Annotation Plugin]
+    K --> L[Upload Coverage Report]
+    L --> M[Ensure 'results' Dir exists]
+    M --> N[Run Integration Tests]
+    N --> O[Upload Test Report]
+  end
+
+  style StepGroup stroke-dasharray: 4 4,stroke:#999
+```
+
+# lint.yml
+
+```mermaid
+flowchart TD
+  A[Trigger: push or PR to main/master] --> B[Job: lint - ubuntu-latest]
+  B --> C
+
+  subgraph StepGroup [ ]
+    direction TB
+    C[Checkout Repository]
+    C --> D[Set Up Python]
+    D --> E[Install Dependencies]
+    E --> F[Run Pylint]
+    F --> G[Run Pyright]
+  end
+
+  style StepGroup stroke-dasharray: 4 4,stroke:#999
+```