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>

Cargo.toml

@@ -70,6 +70,8 @@ pest = "2.8.1"
 pest_derive = "2.8.1"
 dirs = "6.0.0"
 similar = { version = "2.7.0", default-features = false, features = ["text"] }
+ureq = { version = "3", features = ["json"], optional = true }
+os_pipe = { version = "1", optional = true }
 
 [features]
 # Default features include core compression engines and swagger UI
@@ -102,6 +104,9 @@ mcp = ["dep:rmcp"]
 # Swagger UI feature
 swagger = ["dep:utoipa-swagger-ui"]
 
+# Client feature (HTTP client for remote server)
+client = ["dep:ureq", "dep:os_pipe"]
+
 [dev-dependencies]
 tempfile = "3.3.0"
 rand = "0.8.5"