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/tokens.rs

@@ -312,5 +312,6 @@ use crate::meta_plugin::register_meta_plugin;
 fn register_tokens_plugin() {
     register_meta_plugin(MetaPluginType::Tokens, |options, outputs| {
         Box::new(TokensMetaPlugin::new(options, outputs))
-    });
+    })
+    .expect("Failed to register TokensMetaPlugin");
 }