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/args.rs

@@ -165,6 +165,10 @@ pub struct OptionsArgs {
     #[arg(help("Path to file containing JWT secret (requires --server)"))]
     pub server_jwt_secret_file: Option<PathBuf>,
 
+    #[arg(long, env("KEEP_SERVER_MAX_BODY_SIZE"))]
+    #[arg(help("Maximum request body size in bytes (requires --server, default: unlimited)"))]
+    pub server_max_body_size: Option<u64>,
+
     #[cfg(feature = "client")]
     #[arg(long, env("KEEP_CLIENT_URL"), help_heading("Client Options"))]
     #[arg(help("Remote keep server URL for client mode"))]