chore(cleanup): dead code + fnm error-wrapping + nvm Current() test gap from cleanup-feature PR

[dipto0321]

Describe the bug

Small cleanup items found while reviewing the post-upgrade cleanup feature (PR #56 / issue #41), bundled since each is a one-line-to-small fix:

1. Dead code in internal/cli/cleanup.go:166,177: confirm := true is declared and then only ever referenced via _ = confirm — it never actually gates anything (the real control flow is a continue elsewhere). Leftover from a refactor; delete both lines.
2. Dead code in internal/cli/upgrade.go:110: _ = yes immediately after yes has already been fully consumed by the cleanup-flag resolution block above it (lines 94-109). Remove.
3. internal/detector/fnm.go:247 (GlobalNpmPrefix's final error path) drops the underlying os.Stat error entirely:

   return "", fmt.Errorf("fnm global npm prefix for %s not found at %s or %s", v, prefix, filepath.Join(dir, "installation", "lib", "node_modules"))

   No %w, no err2 included, even though every one of its 6 sibling implementations (asdf, mise, n, nodenv, nvm, volta) correctly wraps the underlying OS error with %w in the equivalent function. Low severity (message quality/debuggability only), but worth making consistent.
4. internal/detector/nvm.go's Current() method has zero test coverage at the method level. nvm_test.go only tests parseNVMCurrent (the pure output parser) — there's no TestNVMCurrent_InvokesShell-style test asserting NVM.Current() actually builds and runs the source ... && nvm current script, unlike every other manager (fnm, mise, n, nodenv, asdf, volta all have this test). Given Current() is the safety-critical gate protecting the active version from deletion (see the related "Current() error leaves active version unprotected" issue), this is worth closing.

Severity

Low individually — bundled as one chore PR since none needs its own review cycle.