feat: v2 Phase 1 — foundation, proto schemas, RPC framework, SDK skeleton

New workspace structure with 9 crates. Adds:

- proto/qpq/v1/*.proto: 11 protobuf schemas covering all 33 RPC methods
- quicproquo-proto: dual codegen (capnp legacy + prost v2)
- quicproquo-rpc: QUIC RPC framework (framing, server, client, middleware)
- quicproquo-sdk: client SDK (QpqClient, events, conversation store)
- quicproquo-server/domain/: protocol-agnostic domain types and services
- justfile: build commands

Wire format: [method_id:u16][req_id:u32][len:u32][protobuf] per QUIC stream.
All 151 existing tests pass. Backward compatible with v1 capnp code.

crates/quicproquo-sdk/src/error.rs

@@ -0,0 +1,29 @@
+//! SDK error types.
+
+/// Errors returned by SDK operations.
+#[derive(Debug, thiserror::Error)]
+pub enum SdkError {
+    #[error("not connected to server")]
+    NotConnected,
+
+    #[error("not authenticated — call login() first")]
+    NotAuthenticated,
+
+    #[error("authentication failed: {0}")]
+    AuthFailed(String),
+
+    #[error("conversation not found: {0}")]
+    ConversationNotFound(String),
+
+    #[error("crypto error: {0}")]
+    Crypto(String),
+
+    #[error("RPC error: {0}")]
+    Rpc(#[from] quicproquo_rpc::error::RpcError),
+
+    #[error("storage error: {0}")]
+    Storage(String),
+
+    #[error("{0}")]
+    Other(#[from] anyhow::Error),
+}