Windows: background process spawns flash visible console windows over the UI #2490

Description

On Windows, the desktop app continuously flashes black console windows over the
UI during normal use. Every std::process::Command spawn of a console-subsystem
binary (git, node, ffmpeg, agent CLIs) opens a visible console window unless the
CREATE_NO_WINDOW process-creation flag is set, and none of the background
spawn sites in desktop/src-tauri set it.

Steps to reproduce

1. Build and run the desktop app on Windows (v0.4.23, source build).
2. Open a workspace with a project/repo attached and at least one agent runtime
   installed (e.g. Claude Code or Codex).
3. Watch the screen during idle/normal use.

Expected: no visible console windows.

Actual: console windows flash repeatedly — on git operations, agent runtime
discovery/version probes, auth status probes, and media transcodes. On an
otherwise idle workspace this happens every few seconds (readiness polling).

Affected spawn sites

All background (non-interactive) spawns under desktop/src-tauri/src:

• commands/project_git_exec.rs — run_git (every git invocation)
• commands/agent_discovery/managed_node.rs — managed node --version probe
• commands/agent_model_process.rs — agent models listing
• commands/agent_auth.rs — ACP auth command + Claude subscription login
• commands/media_transcode.rs — ffmpeg command builder (+ test fixture generation)
• commands/relay_reconnect.rs — run_with_timeout
• managed_agents/backend.rs — invoke_provider
• managed_agents/discovery.rs — login-shell probe, auth status probe, codex ACP version probe
• managed_agents/agent_env.rs — env probe tests

Proposed fix

Set creation_flags(CREATE_NO_WINDOW) (0x0800_0000) on these commands,
guarded by #[cfg(windows)] via std::os::windows::process::CommandExt — no
new dependencies, no behavior change on other platforms. Interactive/foreground
spawns are left untouched.

I run this patch locally (9 files, ~116 insertions) and can open a PR —
branch is ready.

Environment

• Windows 11 Pro 25H2 (build 26200)
• Buzz desktop v0.4.23, built from source (MSVC, Rust 1.95.0)