feat(detector): implement Volta detection with mocked tests

[dipto0321]

Summary

Phase 1 manager 3/8: Volta detection. Volta is a real binary at
$VOLTA_HOME/bin/volta (or on PATH) that stores fully-resolved Node
installs on disk under $VOLTA_HOME/tools/image/node/vX.Y.Z/.

What this PR does

Implements the Phase 1 detection surface for the volta manager:

Method	Strategy	Subprocess?
Detect()	PATH lookup via platform.LookupManagerBinary OR <voltaHome>/bin/volta exists on disk	No
Version()	volta --version via runShell, parsed to drop the leading volta if present	Yes
ListInstalled()	Read <voltaHome>/tools/image/node/* directory entries, skip non-dirs and unparseable names	No

Mutation methods (Install, Uninstall, Use, SetDefault,
GlobalNpmPrefix) return a new ErrVoltaNotImplemented sentinel.

Reuses the package-level seams already declared elsewhere:

• runShell (declared in fnm.go) — Volta's volta --version is a
  binary invocation, same shape as FNM.
• listDir (declared in nvm.go) — Volta's installed list comes from
  reading a fixed directory structure, same shape as NVM.

Why this design

• Hybrid Detect mirrors how Volta actually ships: installed via the
  Volta installer it lives at $VOLTA_HOME/bin/volta; installed via
  Homebrew it lives somewhere on $PATH. Either signal is enough.
• Direct disk read for ListInstalled is faster and more
  deterministic than shelling out — and is what Volta itself does
  internally.
• No system sentinel like NVM has. Volta only contains real
  installs.

Notes

• Volta is in maintenance mode (last release v2.0.2 on 2024-12-05,
  README recommends migrating to mise). nodeup still supports it
  because many users haven't migrated yet.
• Skipped the live probe — the user doesn't have Volta installed and
  didn't want test managers installed (see feat(detector): implement NVM detection with mocked tests #2's PR body for the
  precedent). Coverage rests on the 29 mocked tests.

Tests

29 new tests in internal/detector/volta_test.go, all PASS:

• 5 parseVoltaVersion cases (standard, bare, whitespace, empty, ws-only)
• 5 parseVoltaInstalledEntries cases (happy, no-v-prefix, non-dirs,
  unparseable, empty)
• 5 Version cases (success, bare-version, runShell-error, parse-error,
  sub-cases)
• 3 Detect cases (no PATH no home, finds-binary, honors VOLTA_HOME)
• 5 ListInstalled cases (success, no-image-dir, list-dir-error, reads
  the expected path, no-mutation)
• 2 helper tests (voltaHome env-override + whitespace, voltaBinaryPath)
• 1 mutation-stub check
• 1 Detect-doesn't-invoke-runShell guard

Full package: 79 tests (21 FNM + 23 NVM + 29 Volta + 5 registry + 1
Volta-home override). All PASS with -race.

Checklist

• go test -race ./... green
• golangci-lint run ./internal/detector/... clean
• Follows the same seam pattern as feat(detector): implement FNM detection with mocked tests #1 (FNM) and feat(detector): implement NVM detection with mocked tests #2 (NVM)
• GitHub Flow: dedicated branch feat/detector/implement-volta

Closes nothing by itself; advances Phase 1 (detector engine) in
nodeup.md.

[cocogitto-bot]

✔️ e971b68 - Conventional commits check succeeded.