fix(cli): implement nodeup list and add JSON envelope

[dipto0321]

Summary

nodeup list was a stub printing "not yet implemented (Phase 1)". It now does the obvious thing — resolve every detected manager via DetectAll, call each one's ListInstalled(), and render the union either as a JSON envelope (--json) or a one-line-per-manager table. Versions are sorted ascending (semver — pre-release tags order correctly). The currently-active version, when at least one manager answers Current(), is surfaced alongside the listing in both renderers.

A new --manager flag narrows the listing to a single manager (case-insensitive), matching the same flag on nodeup upgrade and nodeup check. Per-manager errors from ListInstalled are captured in the JSON envelope's error field rather than aborting the whole listing, mirroring the soft-fail policy already on check.

Closes #45.

Linked issues

• Closes bug(cli): nodeup list is an unimplemented stub, contradicts README and breaks --json scripting #45

Type of change

• fix — bug fix (PATCH bump) (filling in for the long-broken stub)

Checklist

• Title follows Conventional Commits (fix(scope): subject)
• I ran make ci locally and it passes (fmt, vet, test pass locally; make lint fails on the v2→v1 schema mismatch tracked in build(lint): .golangci.yml v1 schema incompatible with golangci-lint v2 (brew installs v2, make lint fails to even parse config) #62 — pre-existing on main, independent of this fix; CI runs golangci-lint v1.64.8 per .github/workflows/ci.yml and parses the config fine)
• I added or updated tests for the change (new internal/cli/list_test.go — JSON envelope happy-path + omits-on-nil current/error, table renderer empty + mixed-states, --manager resolution helper incl. case-insensitive, formatListVersions, lower, registryNames)
• I updated relevant docs (README, docs/, inline godoc)
• No new linter warnings
• If breaking: I documented the migration path in the PR body and updated CHANGELOG.md (N/A — no CLI surface breaking; previously the command was a no-op stub)

Scope notes

• The shape of nodeup list --json (top-level envelope with installed: [{manager, versions, error?}] and an optional current) is intentionally similar to — but not identical to — nodeup check --json. check reports what nodejs.org offers; list reports what's installed locally. The per-entry field names (manager, versions, error) match so a jq pipeline collecting both commands doesn't have to special-case.
• Versions is rendered as strings (semver) rather than numbers because pre-release tags (20.0.0-rc.1, ...) are not valid JSON numbers and would otherwise be silently dropped on the wire.
• Current is first manager that answers Current() — there is typically one manager-owned node on PATH per machine, so this is a best-effort signal rather than a strict contract. Managers that don't implement Current (e.g., nvm-windows — returns ErrNVMWindowsNotImplemented) are skipped silently.
• I deliberately did NOT plumb Current through the table renderer unconditionally: it prints only when probeable. The empty-state line "No Node.js version manager detected." mirrors check.go's wording exactly.
• The new helper findManagerByName is case-insensitive (matches by lowercased names) so --manager FNM works the same as --manager fnm. The error message lists every detected manager in registration order — registryNames keeps that ordering consistent with the rest of the CLI.

Out of scope (separate issues)

• internal/cli/packages.go still uses context.Background() at line ~141 for Restore — fix(cli): context.Background() used instead of cmd.Context() in packages.go (contextcheck violation) #49.
• Sentinel lifecycle bug (sentinel not cleared on partial failure) — bug(packages): interrupted-upgrade sentinel lifecycle broken (deleted on failure, never cleared after manual restore) #47.
• nodeup packages restore aborts on first failed install — bug(packages): restore aborts on first failed install; migration report never written #46.
• Manifest fetch has no timeout/retry — bug(node): no HTTP timeout/retry/context on nodejs.org manifest fetch; cache write not atomic #48.

Screenshots / output

User-visible behaviour change:

• Before: nodeup list printed nodeup list — not yet implemented (Phase 1) and exited 0.
• After: nodeup list enumerates installed versions across every detected manager:

  
    fnm:    20.18.0, 22.11.0
    volta:  18.20.4
    Active: 22.11.0
  

  And with --manager fnm:

  
    fnm:    20.18.0, 22.11.0
    Active: 22.11.0
  

  And with --json:

  {
    "installed": [
      {"manager": "fnm",   "versions": ["20.18.0", "22.11.0"]},
      {"manager": "volta", "versions": ["18.20.4"]}
    ],
    "current": "22.11.0"
  }

Verification path: nodeup list, nodeup list --json, nodeup list --manager fnm on a machine with at least one manager installed; go test -race ./internal/cli/... for the unit-test surface.

[cocogitto-bot]

✔️ 61400dd - Conventional commits check succeeded.