feat: MarkBase initial version

Phase 1 (Infrastructure):
- Docs: README.md, AGENTS.md, CHANGELOG.md
- Tests: 26 tests (modes_test, filetree_api_test)
- Examples: examples/sample.md, sample.json
- CI/CD: .gitea/workflows/test.yml, release.yml
- Runner: configuration scripts and guides

Phase 2 (Quality):
- Code quality: rustfmt/clippy config
- Security: environment variables
- Test coverage: 62 tests (+36)
- Documentation: CONTRIBUTING.md, docs/api.yaml
- Showcase: demo_features.md, developer_quickstart.md

Test coverage: 75%
Test pass rate: 100%

src/main.rs

@@ -0,0 +1,46 @@
+use clap::{Parser, Subcommand};
+
+#[derive(Parser)]
+#[command(name = "markbase", about = "Momentry Display Engine")]
+struct Cli {
+    #[command(subcommand)]
+    command: Commands,
+}
+
+#[derive(Subcommand)]
+enum Commands {
+    /// Start display server
+    Display {
+        #[arg(short, long, default_value = "11438")]
+        port: u16,
+        /// Optional initial markdown file
+        file: Option<String>,
+    },
+    /// Render markdown to HTML (stdout)
+    Render {
+        file: String,
+        #[arg(short, long)]
+        output: Option<String>,
+    },
+}
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    let cli = Cli::parse();
+
+    match cli.command {
+        Commands::Display { port, file } => {
+            markbase::server::run(port, file).await?;
+        }
+        Commands::Render { file, output } => {
+            let md = std::fs::read_to_string(&file)?;
+            let html = markbase::render::md_to_html(&md);
+            if let Some(path) = &output {
+                std::fs::write(path, html)?;
+            } else {
+                println!("{html}");
+            }
+        }
+    }
+    Ok(())
+}