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/modes/client/save.rs

@@ -92,7 +92,7 @@ pub fn mode(
         let digest = format!("{:x}", hasher.finalize());
 
         // Set shared state for main thread
-        let mut shared = shared_reader.lock().unwrap();
+        let mut shared = shared_reader.lock().expect("client save mutex poisoned");
         *shared = (total_bytes, digest.clone());
 
         Ok((total_bytes, digest))
@@ -135,7 +135,7 @@ pub fn mode(
 
     // Read results from shared state
     let (uncompressed_size, digest) = {
-        let shared = shared.lock().unwrap();
+        let shared = shared.lock().expect("client save mutex poisoned");
         shared.clone()
     };