Improve missing install tool guidance

[jiahao6635]

Summary

• detect common missing-executable errors emitted by Go, env, and Unix shells
• name the missing command and explain that it must be installed and available in PATH
• preserve the existing generic guidance for unrelated installation failures
• add offline regression tests, including the Windows PATH spelling

Testing

• go test -race ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.4 run
• go build -o /tmp/lk ./cmd/lk
• generated fish completion matches autocomplete/fish_autocomplete

Fixes #912

[CLAassistant]

All committers have signed the CLA.

[Propconnect-ai]

Ran the Windows verification on Windows 11.

Result: could not execute the tests. go test ./cmd/lk/... fails to build on Windows with the MinGW/GCC toolchain.

Environment: Windows 11, Go 1.26.5, GCC 16.2.0 (MSYS2 UCRT64), CGO_ENABLED=1

Steps taken: installed MSYS2 + mingw-w64-ucrt-x86_64-gcc, set CGO_CXXFLAGS_ALLOW=.* (needed for -fms-extensions), ran git submodule update --init --recursive for pkg/portaudio/pa_src.

Failure:

platform_thread_types.cc:111:3: error: '__try' was not declared in this scope
platform_thread_types.cc:114:5: error: '__except' was not declared in this scope

__try/__except are MSVC structured exception handling; GCC doesn't support them. Since cmd/lk imports pkg/console → pkg/apm, the whole package fails to build, so the new tests in cmd/lk/app_test.go can't run on Windows without MSVC.

Flagging because the PR adds a Windows PATH regression test — worth knowing it can currently only be exercised on Linux CI. Happy to retry with MSVC if that's the supported Windows path.