Windows: hide background Dev MCP shell windows

[paritosh921]

Summary

• Start background Dev MCP shell commands without creating a visible Windows console window.
• Keep Windows shell-environment tests serial so they do not race with real shell-command tests.
• Allow a longer startup window for Git Bash in the basic shell test on busy Windows workers.

Root cause

Buzz captures stdout and stderr for background shell tool calls, but Windows still creates a visible console window for console applications such as Git Bash unless the process is launched with CREATE_NO_WINDOW.

Validation

• cargo fmt --all -- --check
• cargo test -p buzz-dev-mcp
• cargo clippy -p buzz-dev-mcp --all-targets -- -D warnings
• Desktop Rust test suite
• pnpm typecheck
• pnpm test
• pnpm build
• Manual Windows test: navigation and background agent activity did not open Git Bash windows.