refactor: streaming, security hardening, and MCP removal

Major overhaul of server architecture and security posture:

- Streaming: Unified all I/O through PIPESIZE (8192-byte) buffers.
  POST bodies stream via MpscReader through the save pipeline. GET
  content streams from disk via decompression to client. Removed
  save_item_with_reader, get_item_content_info, ChannelReader.
  413 responses keep partial items (nonfatal by design).

- Security: XSS protection in all HTML pages via html_escape crate.
  Security headers middleware (nosniff, frame deny, referrer policy).
  CORS tightened to explicit headers. Input validation for tags
  (256 chars), metadata (128/4096), pagination (10k cap). Config
  file reads use from_utf8_lossy. Generic error messages in HTML.
  Diff endpoint has 10 MB per-item cap. max_body_size config option.

- Panics eliminated: Path unwraps → proper error propagation.
  Mutex unwraps → map_err (registries) / expect with message (local).

- MCP removed: Deleted all MCP code, rmcp dependency, mcp feature.

- Docs: Updated README, DESIGN, AGENTS to reflect all changes.

src/config.rs

@@ -152,6 +152,7 @@ pub struct ServerConfig {
     pub cert_file: Option<PathBuf>,
     pub key_file: Option<PathBuf>,
     pub cors_origin: Option<String>,
+    pub max_body_size: Option<u64>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]
@@ -256,7 +257,11 @@ impl Settings {
         // Override with CLI args
         if let Some(dir) = &args.options.dir {
             debug!("CONFIG: Overriding dir with CLI arg: {dir:?}");
-            config_builder = config_builder.set_override("dir", dir.to_str().unwrap())?;
+            config_builder = config_builder.set_override(
+                "dir",
+                dir.to_str()
+                    .ok_or_else(|| anyhow::anyhow!("non-UTF-8 directory path"))?,
+            )?;
         }
 
         if args.options.human_readable {
@@ -316,6 +321,10 @@ impl Settings {
                 .set_override("server.key_file", server_key.to_string_lossy().as_ref())?;
         }
 
+        if let Some(max_body_size) = args.options.server_max_body_size {
+            config_builder = config_builder.set_override("server.max_body_size", max_body_size)?;
+        }
+
         if let Some(compression) = &args.item.compression {
             config_builder =
                 config_builder.set_override("compression_plugin.name", compression.as_str())?;
@@ -486,10 +495,10 @@ impl Settings {
             // First check for password_file
             if let Some(password_file) = &server.password_file {
                 debug!("CONFIG: Reading password from file: {password_file:?}");
-                let password = fs::read_to_string(password_file)
-                    .with_context(|| format!("Failed to read password file: {password_file:?}"))?
-                    .trim()
-                    .to_string();
+                let password = fs::read(password_file)
+                    .with_context(|| format!("Failed to read password file: {password_file:?}"))?;
+                let end = password.len().min(4096);
+                let password = String::from_utf8_lossy(&password[..end]).trim().to_string();
                 return Ok(Some(password));
             }
 
@@ -521,12 +530,11 @@ impl Settings {
             // First check for jwt_secret_file
             if let Some(jwt_secret_file) = &server.jwt_secret_file {
                 debug!("CONFIG: Reading JWT secret from file: {jwt_secret_file:?}");
-                let secret = fs::read_to_string(jwt_secret_file)
-                    .with_context(|| {
-                        format!("Failed to read JWT secret file: {jwt_secret_file:?}")
-                    })?
-                    .trim()
-                    .to_string();
+                let secret = fs::read(jwt_secret_file).with_context(|| {
+                    format!("Failed to read JWT secret file: {jwt_secret_file:?}")
+                })?;
+                let end = secret.len().min(4096);
+                let secret = String::from_utf8_lossy(&secret[..end]).trim().to_string();
                 return Ok(Some(secret));
             }