refactor(detector): add context.Context to Manager.ListInstalled and Current

[dipto0321]

Summary

Mechanical refactor that adds context.Context as the first
parameter to the Manager interface's two read methods:
ListInstalled and Current. The other interface methods
(Install, Uninstall, Use, SetDefault, GlobalNpmPrefix,
Version) are intentionally out of scope.

Why

The Manager interface was first laid out without ctx on any
method. The omission became user-visible when the post-upgrade
cleanup feature (PR #56 / issue #41) added Current() and shipped
across all 8 manager implementations: a Ctrl-C mid-nodeup upgrade
would still wait for the underlying manager list / fnm list /
nvm current subprocess to finish before returning, because
cancellation had no path into the read paths. Install /
Uninstall already had a cancellation story (the platform lock +
interrupted-upgrade sentinel from #46), so they aren't part of
this PR.

What

• internal/detector/detector.go — interface signatures:
  • ListInstalled(ctx context.Context) ([]semver.Version, error)
  • Current(ctx context.Context) (semver.Version, error)
  • Both methods got a short godoc note pointing at cmd.Context()
    as the source of ctx.
• 8 concrete implementations in internal/detector/*.go:
  fnm.go, nvm.go, volta.go, asdf.go, mise.go, n.go,
  nodenv.go, nvm_windows.go. Each method signature updated;
  each runShell(context.Background(), ...) /
  runScript(context.Background(), ...) inside the bodies
  replaced with the new ctx parameter. nvm_windows.go's
  Current is the lone non-shell body (returns the
  ErrNVMWindowsNotImplemented sentinel) — it just gets a
  discarded _ context.Context parameter.
• 6 production call-sites in internal/cli/{check,list, upgrade,packages}.go now pass cmd.Context(). packages.go's
  helper getCurrentVersion(m) becomes
  getCurrentVersion(ctx, m); its sole caller at line 50 passes
  cmd.Context().
• 3 mock/stub Manager types in *_test.go:
  fakeManager (system_node_test.go), listTestStubManager
  (cli/list_test.go), stubManager (cli/cleanup_test.go). Each
  takes a discarded _ context.Context on ListInstalled and
  Current; bodies unchanged.
• 39 test call-sites in the per-manager *_test.go files now
  pass t.Context() (Go 1.24 added t.Context(); the project's
  go.mod is on go 1.24).

Linter verification

contextcheck is enabled in .golangci.yml:48 and (per the
exploration) the _test\.go exclusion only covers errcheck /
gocritic, not contextcheck. So production call-sites with a
cobra cmd.Context() in scope must use it — not
context.Background(). All 6 production call-sites do so. make lint is green.

Linked issues

Closes #53

Type of change

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

Checklist

• Title follows Conventional Commits (refactor(scope): subject)
• I ran make ci locally and it passes (go vet,
  golangci-lint run (including contextcheck), go test -race -coverprofile=coverage.out ./... all green; coverage
  59.6% unchanged)
• I added or updated tests for the change (no new tests; all
  39 existing test call-sites updated to the new signature, no
  behavior change)
• I updated relevant docs (no user-facing docs change;
  CHANGELOG.md gets a ### Changed 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

Files touched (24)

Interface + 8 managers (10 files in internal/detector/):
detector.go, fnm.go, nvm.go, volta.go, asdf.go,
mise.go, n.go, nodenv.go, nvm_windows.go, and the
production test stub system_node_test.go.

Production call-sites (4 files in internal/cli/):
check.go, list.go, upgrade.go, packages.go.

Test stubs + call-sites (8 files in *_test.go):
cli/list_test.go, cli/cleanup_test.go, and the 6 per-manager
test files in internal/detector/: asdf_test.go, fnm_test.go,
mise_test.go, n_test.go, nodenv_test.go, nvm_test.go,
nvm_windows_test.go, volta_test.go (one of those is the 8th —
the stubManager in cli/cleanup_test.go is the 9th).

Changelog: CHANGELOG.md.

[cocogitto-bot]

✔️ 987aac6 - Conventional commits check succeeded.