feat: add client mode with streaming support

Add client mode enabling the keep CLI to connect to a remote keep
server over HTTP. Local plugins (compression, meta, filters) run on
the client; the server stores/retrieves binary blobs.

Architecture:
- Client save uses 3-thread streaming pipeline: reader thread (stdin
  → tee/stdout → hash → compress), OS pipe, streamer thread (pipe →
  chunked HTTP POST). Memory usage is O(PIPESIZE) regardless of data
  size.
- Server accepts compress=false, meta=false, decompress=false query
  params for granular control of server-side processing.
- Streaming body handling on server via async channel → sync reader
  bridge (ChannelReader).

Key additions:
- src/client.rs: KeepClient with post_stream() for chunked upload
- src/modes/client/: save, get, list, info, delete, diff, status
- --client-url / KEEP_CLIENT_URL configuration
- --client-password / KEEP_CLIENT_PASSWORD for auth
- os_pipe dependency for zero-copy pipe streaming

Co-Authored-By: andrew/openrouter/hunter-alpha <noreply@opencode.ai>

src/lib.rs

@@ -40,6 +40,9 @@ pub mod meta_plugin;
 pub mod modes;
 pub mod services;
 
+#[cfg(feature = "client")]
+pub mod client;
+
 // Re-export Args struct for library usage
 pub use args::Args;
 // Re-export PIPESIZE constant