fix: make managed-agent spawn and teardown portable to Windows

[wpfleger96]

Several managed-agent spawn, teardown, and shim paths were #[cfg(unix)]-only and silently no-op (or returned a falsey stub) on Windows, breaking the desktop build four ways. All four fixes share one Windows-portability theme.

#2 / #4 — MCP PermissionDenied on C:\Windows

buzz-agent spawns the MCP with cmd.env_clear() then re-adds only an allowlist (PASSTHROUGH_ENV) that had TMPDIR but none of the Windows temp/profile vars. Stripped of TMP/TEMP/USERPROFILE, std::env::temp_dir() falls all the way back to C:\Windows, where Shim::install() can't create its tempdir → PermissionDenied (os error 5) → every MCP init dies.

• crates/buzz-agent/src/mcp.rs: cfg-gated PASSTHROUGH_ENV_WINDOWS adds TMP, TEMP, USERPROFILE (load-bearing for temp_dir(); USERPROFILE is the always-set floor) plus LOCALAPPDATA, APPDATA (child-tool config — git, etc.).

The shim install path had two more Unix-isms that would silently break buzz/git shell-outs even after temp_dir() was fixed:

• crates/buzz-dev-mcp/src/shim.rs: PATH was built with a hardcoded : separator — now std::env::split_paths/join_paths (platform separator). The #[cfg(not(unix))] multicall copy dropped the .exe extension PATHEXT needs to treat the file as runnable — now appended.
• crates/buzz-dev-mcp/src/lib.rs: multicall dispatch matches on file_stem() instead of file_name(), so the .exe copies (rg.exe, buzz.exe, ...) route to the correct match arm.

#1 — stray buzz-acp.exe console + orphaned process tree

The buzz-acp child spawned with no CREATE_NO_WINDOW flag (a console window popped and lingered), and the non-unix stop path was Child::kill(), which kills only the harness and orphans the 24 agent workers + MCP servers it spawned.

• desktop/src-tauri/src/managed_agents/process_lifecycle.rs (new, #[cfg(windows)]): a Win32 Job Object (JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE) owns the harness tree and reaps it when the handle drops — the Windows mirror of the Unix process_group(0) teardown. The after-restart path (PID-only, no handle) falls back to taskkill /T.
• desktop/src-tauri/src/managed_agents/runtime.rs: CREATE_NO_WINDOW at the spawn site; stop_managed_agent_process drops the job handle on Windows (falls back to Child::kill() if assignment failed); terminate_process delegates to taskkill_tree; the augmented-PATH builder also moves off the hardcoded : separator.
• desktop/src-tauri/src/managed_agents/types.rs: ManagedAgentProcess carries the #[cfg(windows)] job handle.

The Job Object dies with its owner, so a co-located sibling instance's agents are never affected — no env-read PEB walk or identity-matched sweep is needed.

#4 — agent_cmd=goose acp, "program not found"

With an empty managed-agents.json, a freshly-created agent fell through to the platform default goose, which isn't on PATH on a stock Windows install → all 24 workers failed with program not found.

• desktop/src-tauri/src/managed_agents/discovery.rs: new default_agent_command() catalog-resolves the bundled buzz-agent — the same shape mesh_llm::preset already uses, so the default can't drift from the provider definition. buzz-agent takes no acp arg, so there's no arg leakage.
• desktop/src-tauri/src/commands/agents.rs, types.rs: create path uses the resolver; the dead DEFAULT_AGENT_COMMAND = "goose" const is removed.

#3 — "Check for Updates" silently does nothing

v0.3.24 is the latest tag, so the build is genuinely up-to-date. But when the updater plugin is unavailable, the hook collapsed to idle, re-rendering the same "Check for Updates" button — indistinguishable from a no-op.

• desktop/src/features/settings/hooks/use-updater.ts: the unavailable branch now console.warns (so the firing branch is diagnosable in the Windows app log) and sets a visible unavailable state instead of idle.
• desktop/src/features/settings/UpdateChecker.tsx: renders a clear "Automatic updates aren't available on this build" row.

Verification

Verified on the macOS dev host: just desktop-check, just desktop-test (922 TS tests), just desktop-tauri-test (547 Rust tests incl. the new default_agent_command test and windows_passthrough_includes_temp_dir_vars), cargo clippy/cargo fmt clean across the touched workspace crates and the Tauri crate.

The #[cfg(windows)] Job Object, CREATE_NO_WINDOW, shim install, and the updater branch are not exercised by the host test suite and were not type-checked by a Windows compiler (no local MSVC toolchain). They were verified by inspecting windows-sys 0.61 symbol signatures. Windows CI / a manual run on Will's box is the real gate for those surfaces.

A separate minor noted but intentionally not addressed: devtools is unreachable on release Windows builds because windows_subsystem="windows" non-debug compiles the inspector out.