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.

Dockerfile

@@ -23,7 +23,7 @@ RUN cargo fetch --target x86_64-unknown-linux-musl
 # magic feature excluded (requires shared libmagic; fallback uses `file` command)
 COPY src/ src/
 RUN cargo build --release --target x86_64-unknown-linux-musl \
-    --no-default-features --features lz4,gzip,server,mcp,swagger,client,tls \
+    --no-default-features --features lz4,gzip,server,swagger,client,tls \
     && strip target/x86_64-unknown-linux-musl/release/keep
 
 # Runtime stage - scratch since binary is fully static