perf(deps): remove mcp-bridge from default features to reduce release binary deps

Move tauri-plugin-mcp-bridge behind opt-in `mcp` feature flag instead
of default. Eliminates duplicate objc2 0.5/0.6, block2 0.5/0.6,
thiserror v1/v2, and other transitive deps from release builds.

Dev workflow: use `task tauri:dev:mcp` to enable MCP bridge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

Cargo.lock

@@ -81,7 +81,7 @@ tracing-appender = "0.2"
 dirs = "6"
 
 [features]
-default = ["mcp"]
+default = []
 devtools = ["dep:tauri-plugin-devtools", "tauri/devtools"]
 mcp = ["dep:tauri-plugin-mcp-bridge"]