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/args.rs

@@ -141,6 +141,16 @@ pub struct OptionsArgs {
     #[arg(help("Password hash for server authentication (requires --server)"))]
     pub server_password_hash: Option<String>,
 
+    #[cfg(feature = "client")]
+    #[arg(long, env("KEEP_CLIENT_URL"), help_heading("Client Options"))]
+    #[arg(help("Remote keep server URL for client mode"))]
+    pub client_url: Option<String>,
+
+    #[cfg(feature = "client")]
+    #[arg(long, env("KEEP_CLIENT_PASSWORD"), help_heading("Client Options"))]
+    #[arg(help("Password for remote keep server authentication"))]
+    pub client_password: Option<String>,
+
     #[arg(
         long,
         help("Force output even when binary data would be sent to a TTY")