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/config.rs

@@ -153,6 +153,12 @@ pub struct CompressionPluginConfig {
     pub name: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ClientConfig {
+    pub url: Option<String>,
+    pub password: Option<String>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 #[cfg_attr(feature = "server", derive(utoipa::ToSchema))]
 pub struct MetaPluginConfig {
@@ -184,6 +190,12 @@ pub struct Settings {
     pub server: Option<ServerConfig>,
     pub compression_plugin: Option<CompressionPluginConfig>,
     pub meta_plugins: Option<Vec<MetaPluginConfig>>,
+    pub client: Option<ClientConfig>,
+    // Non-serializable fields populated from CLI args
+    #[serde(skip)]
+    pub client_url: Option<String>,
+    #[serde(skip)]
+    pub client_password: Option<String>,
 }
 
 impl Settings {
@@ -394,6 +406,21 @@ impl Settings {
                     settings.dir = default_dir;
                 }
 
+                // Populate client settings from CLI args and config
+                #[cfg(feature = "client")]
+                {
+                    settings.client_url = args
+                        .options
+                        .client_url
+                        .clone()
+                        .or_else(|| settings.client.as_ref().and_then(|c| c.url.clone()));
+                    settings.client_password = args
+                        .options
+                        .client_password
+                        .clone()
+                        .or_else(|| settings.client.as_ref().and_then(|c| c.password.clone()));
+                }
+
                 debug!("CONFIG: Final settings: {settings:?}");
                 Ok(settings)
             }