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/modes/client/list.rs

@@ -0,0 +1,55 @@
+use crate::client::KeepClient;
+use crate::modes::common::{OutputFormat, format_size, settings_output_format};
+use clap::Command;
+use log::debug;
+
+pub fn mode(
+    client: &KeepClient,
+    _cmd: &mut Command,
+    settings: &crate::config::Settings,
+    tags: &[String],
+) -> Result<(), anyhow::Error> {
+    debug!("CLIENT_LIST: Listing items via remote server");
+
+    let items = client.list_items(tags, "newest", 0, 100)?;
+
+    let output_format = settings_output_format(settings);
+
+    match output_format {
+        OutputFormat::Json => {
+            println!("{}", serde_json::to_string_pretty(&items)?);
+        }
+        OutputFormat::Yaml => {
+            println!("{}", serde_yaml::to_string(&items)?);
+        }
+        OutputFormat::Table => {
+            use comfy_table::{Table, presets::UTF8_FULL};
+
+            let mut table = Table::new();
+            table.load_preset(UTF8_FULL);
+
+            // Header
+            let headers = ["ID", "Time", "Size", "Compression", "Tags"];
+            table.set_header(headers.iter().map(|h| h.to_string()).collect::<Vec<_>>());
+
+            for item in &items {
+                let size_str = item
+                    .size
+                    .map(|s| format_size(s as u64, settings.human_readable))
+                    .unwrap_or_default();
+
+                table.add_row(vec![
+                    item.id.to_string(),
+                    item.ts.clone(),
+                    size_str,
+                    item.compression.clone(),
+                    item.tags.join(", "),
+                ]);
+            }
+
+            println!("{table}");
+        }
+    }
+
+    Ok(())
+}