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/meta_plugin/mod.rs

@@ -578,11 +578,15 @@ static META_PLUGIN_REGISTRY: Lazy<Mutex<HashMap<MetaPluginType, PluginConstructo
 ///
 /// * `meta_plugin_type` - The type of the meta plugin to register.
 /// * `constructor` - The constructor function for creating plugin instances.
-pub fn register_meta_plugin(meta_plugin_type: MetaPluginType, constructor: PluginConstructor) {
+pub fn register_meta_plugin(
+    meta_plugin_type: MetaPluginType,
+    constructor: PluginConstructor,
+) -> anyhow::Result<()> {
     META_PLUGIN_REGISTRY
         .lock()
-        .unwrap()
+        .map_err(|e| anyhow::anyhow!("plugin registry poisoned: {e}"))?
         .insert(meta_plugin_type, constructor);
+    Ok(())
 }
 
 pub fn get_meta_plugin(
@@ -590,7 +594,9 @@ pub fn get_meta_plugin(
     options: Option<std::collections::HashMap<String, serde_yaml::Value>>,
     outputs: Option<std::collections::HashMap<String, serde_yaml::Value>>,
 ) -> anyhow::Result<Box<dyn MetaPlugin>> {
-    let registry = META_PLUGIN_REGISTRY.lock().unwrap();
+    let registry = META_PLUGIN_REGISTRY
+        .lock()
+        .map_err(|e| anyhow::anyhow!("plugin registry poisoned: {e}"))?;
     if let Some(constructor) = registry.get(&meta_plugin_type) {
         return Ok(constructor(options, outputs));
     }