refactor(core): share process-tree kill helpers across tool crates (#853)

Five crates (tools, build, vcs, github, docker) each carried their own copy of killing a child and what it started, and only the tools installer killed the whole tree on Windows, so cancelling git, gh, docker or Maven/Gradle there left descendants running. ide-core now provides own_group, kill_tree and terminate_tree (process-group signals on Unix, taskkill /T /F on Windows) and the copies are removed. Unix behaviour is unchanged; the Windows path was not run locally.

Co-authored-by: bograh <[email protected]>

crates/ide-github/Cargo.toml

@@ -13,6 +13,3 @@ serde_json.workspace = true
 specta.workspace = true
 tracing.workspace = true
 which = "8"
-
-[target.'cfg(unix)'.dependencies]
-libc = "0.2"