chore(detector): wrap fnm GlobalNpmPrefix error; add NVM.Current tests

[dipto0321]

Summary

Bundles the two remaining cleanup-feature debris items from issue
#61. Two of the four items called out in the issue body were
already removed by PR #95 / issue #76 — cleanup.go's
confirm := true / _ = confirm dead lines and upgrade.go's
_ = yes line — so this PR focuses on the items still standing:

1. internal/detector/fnm.go's GlobalNpmPrefix final error path
   now wraps the underlying os.Stat error with %w. The
   pre-fix line discarded err2 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"))

   Every sibling implementation (asdf, mise, n, nodenv, nvm, volta)
   correctly uses %w in the equivalent function. Low-severity
   consistency fix.

2. NVM.Current() now has full method-level test coverage.
   internal/detector/nvm_test.go previously only exercised the
   pure output parser parseNVMCurrent, but not NVM.Current()
   itself — every other manager's Current() has a
   Test<X>Current_InvokesShell-style test (fnm, asdf, mise,
   nodenv, volta). Three new tests close the gap:

   • TestNVMCurrent_InvokesShell — pins the production script
     shape (sources nvm.sh, runs nvm current, parses output).
   • TestNVMCurrent_PropagatesShellError — pins the wrapped-
     error contract (runScript failure surfaces with %w).
   • TestNVMCurrent_SystemIsError — pins the end-to-end "system"
     handling (nvm current printing "system" surfaces an error,
     not just at the parse layer).

Linked issues

Closes #61

Type of change

• chore — maintenance / dependency bump
• feat — new feature (MINOR bump)
• fix — bug fix (PATCH bump)
• refactor — no behavior change
• perf — performance improvement
• test — tests only
• docs — documentation only
• ci — CI/CD only
• build — build system only
• Breaking change (MAJOR bump) — explain below

Checklist

• Title follows Conventional Commits (chore(scope): subject)
• I ran make ci locally and it passes (golangci-lint run,
  go vet, go test -race ./... all green; detector
  coverage rose from 76.8% → 77.5%)
• I added or updated tests for the change (3 new tests for
  NVM.Current(); GlobalNpmPrefix is exercised by the
  existing TestFNM_* matrix)
• I updated relevant docs (no user-facing change; CHANGELOG.md
  gets a ### Fixed entry under [Unreleased])
• No new linter warnings (make lint green)
• If breaking: I documented the migration path in the PR body
  and updated CHANGELOG.md

Notes

Items #1 (confirm := true / _ = confirm) and #2 (_ = yes)
from issue #61's body were already fixed by PR #95 / issue #76
when that PR restructured the per-version prompt loop. Those
removals are noted in the PR body / CHANGELOG entry but no
new code touches them in this branch.

Files touched

• internal/detector/fnm.go — GlobalNpmPrefix's final error
  path wraps the discarded err2 with %w.
• internal/detector/nvm_test.go — adds three tests for
  NVM.Current(): invocation, error propagation, and the
  end-to-end "system" branch.
• CHANGELOG.md — ### Fixed entry under [Unreleased]
  documenting the wrap and test additions with chore(cleanup): dead code + fnm error-wrapping + nvm Current() test gap from cleanup-feature PR #61 reference.

[cocogitto-bot]

✔️ 6db4199 - Conventional commits check succeeded.