feat: add check_update and update tools with background auto-check

On the first tool call of each session, a background task fires once
and fetches the latest release tag from the GitHub API (5 s timeout).
The result is cached in a static OnceLock for the lifetime of the
process — no repeated network calls.

check_update: returns cached result instantly, or fetches on demand
  if the background task hasn't completed yet.

update: downloads the latest tarball for the current platform, extracts
  the binary, and atomically replaces the running executable via a
  temp-file rename. Requires MCP reconnect to activate new version.

Platform support: macos-aarch64, macos-x86_64, linux-x86_64.
Unsupported platforms return a build-from-source hint.

Tool count: 87 → 89. README and MCP_TOOLS.md updated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/tools/definitions/mod.rs

@@ -63,6 +63,8 @@ pub fn get_tools_list() -> Value {
         system::tool_list_symbols(),
         system::tool_healthcheck(),
         system::tool_get_active_account(),
+        system::tool_check_update(),
+        system::tool_update(),
         // Utility (8 tools)
         utility::tool_check_symbol_data_status(),
         utility::tool_get_backtest_history(),