feat: Add documentation files including Code of Conduct, Contributing guidelines, and Security policy; update Markdown linting configuration and scripts

docs/api/client-articles/index.md

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+# Client Articles
+
+Articles related to the EchoHub TUI client built with Terminal.Gui v2.
+
+## Topics
+
+- Terminal.Gui v2 patterns and conventions
+- Theme system and customization
+- Command system reference
+- Configuration management

docs/api/client-articles/toc.yml

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+- name: Overview
+  href: index.md

docs/api/core-articles/index.md

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+# Core Articles
+
+Articles related to the EchoHub.Core shared library.
+
+## Topics
+
+- Data models and DTOs
+- SignalR contract interface
+- Validation constants and shared rules

docs/api/core-articles/toc.yml

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+- name: Overview
+  href: index.md

docs/api/index.md

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+# API Reference
+
+Browse the generated API documentation for each EchoHub project.
+
+## Projects
+
+### [Client](client/EchoHub.Client.yml)
+
+Terminal.Gui v2 TUI application -- UI components, services, themes, and configuration.
+
+### [Core](core/EchoHub.Core.Constants.yml)
+
+Shared library -- DTOs, models, constants, and the SignalR client contract.
+
+### [Server](server/EchoHub.Server.Controllers.yml)
+
+ASP.NET Core server -- controllers, hubs, authentication, and data access.

docs/api/server-articles/index.md

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+# Server Articles
+
+Articles related to the EchoHub server built with ASP.NET Core.
+
+## Topics
+
+- Authentication and JWT tokens
+- SignalR hub and real-time messaging
+- File upload and validation
+- Rate limiting configuration
+- Database schema and migrations

docs/api/server-articles/toc.yml

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+- name: Overview
+  href: index.md

docs/api/toc.yml

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+- name: Client
+  items:
+    - name: API Reference
+      href: ../_api_meta/client/toc.yml
+    - name: Articles
+      href: client-articles/
+- name: Core
+  items:
+    - name: API Reference
+      href: ../_api_meta/core/toc.yml
+    - name: Articles
+      href: core-articles/
+- name: Server
+  items:
+    - name: API Reference
+      href: ../_api_meta/server/toc.yml
+    - name: Articles
+      href: server-articles/