feat(config:install): improve destination directory detection

[miguelsanchez-upsun]

Summary

• Expand the bin-dir allowlist with package-manager-adjacent locations (Scoop on Windows) and honor XDG_BIN_HOME.
• Prefer the bin directory that already contains the running executable so the alt config installs alongside it; fall back to the first writable PATH entry.
• Skip Homebrew-managed directories (/opt/homebrew/bin on macOS, /home/linuxbrew/.linuxbrew/bin on Linux): a user who installed via Homebrew gets the alt in /usr/local/bin or ~/.local/bin instead of polluting the brew prefix.
• Honor XDG_CONFIG_HOME when locating the config directory (previously ignored on macOS and Windows), even if the target does not yet exist.
• Make -y skip the confirmation prompt (previously neither -y nor --no-interaction did — the prompt only checked stdin TTY).
• Use access(2) for the bin-dir writability check on Unix instead of probing with a temp file.

[Copilot]

Pull request overview

Improves config:install destination directory selection for alt config/executable installs and aligns interactive behavior with --no-interaction / -y.

Changes:

• Honor XDG_CONFIG_HOME when computing the alt config directory.
• Rework FindBinDir() with an expanded per-OS allowlist, writability checks, and preference to co-locate with the running executable (incl. symlink resolution).
• Make config:install confirmation prompt respect no-interaction (so -y/--yes skips it).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
internal/config/alt/fs.go	Adds XDG handling for config dir; overhauls bin-dir selection with allowlist, writability probing, and executable co-location logic.
internal/config/alt/fs_test.go	Refactors and significantly expands bin-dir selection tests; updates config-dir tests for new resolution behavior.
commands/config_install.go	Skips confirmation prompt when no-interaction is enabled (including via -y/--yes).

Comments suppressed due to low confidence (1)

commands/config_install.go:110

• This change makes -y/--yes (via no-interaction) skip the confirmation prompt, but commands/config_install_test.go doesn't currently exercise the interactive-confirmation path (most test environments have non-interactive stdin). Consider adding a unit test that forces terminal.Stdin to interactive and verifies that the prompt is skipped when viper.GetBool("no-interaction") is true (may require injecting/stubbing the confirmation function for testability).

	if terminal.Stdin.IsInteractive() && !viper.GetBool("no-interaction") {
		if !terminal.AskConfirmation("Are you sure you want to continue?", true) {
			os.Exit(1)
		}
		cmd.PrintErrln()

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[Copilot]

TestFindBinDir is now skipped entirely on Windows, but this PR introduces/changes Windows-specific allowlist entries (e.g., Scoop shims). Consider adding Windows-only tests (e.g., via _test.go with //go:build windows) to cover the Windows branch of binDirAllowlist() and FindBinDir() behavior.

[miguelsanchez-upsun]

Don't think this is needed, there's no Windows-specific logic to exercise beyond what the existing branches do on Linux/macOS.

[pjcdawkins]

Hm I think actually we should never be saving our own binaries to the HomeBrew path: it should be owned entirely via HomeBrew.