feat: bootstrap foundation application

content/getting-started.md

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+# Getting Started
+
+Welcome to **MD Hub Secure**.
+
+See [[Architecture Overview]] for the system shape and [[Project Plan]] for the milestone roadmap.
+
+## Features
+
+- Wiki links
+- GitHub-flavored Markdown
+- Tags like #foundation and #security
+- Admonitions
+- Mermaid diagrams
+- Math delimiters for later KaTeX hydration
+
+> [!NOTE]
+> This is a representative admonition block.
+> It should render with a strong callout style.
+
+```mermaid
+flowchart TD
+  A[Markdown] --> B[Renderer]
+  B --> C[Secure HTML]
+```
+
+Inline math: $E = mc^2$
+
+Block math:
+
+$$
+\int_0^1 x^2 dx = \frac{1}{3}
+$$
+