feat(detector): implement NVM detection with mocked tests

[dipto0321]

feat(detector): implement NVM detection with mocked tests

Implements manager #2 of 8 for Phase 1 (detection engine). Closes the
NVM stub that has been living in internal/detector/nvm.go since
scaffolding.

What this PR does

Replaces the zero-value stub methods on NVM with real
Detect / Version / ListInstalled implementations, and adds 23
unit tests covering each one.

Why NVM is special

NVM is a shell function, not a binary. It only exists after
source ~/.nvm/nvm.sh. There is no nvm executable on PATH. This
breaks the simple which fnm && fnm --version pattern that works
for every other manager.

Per nodeup.md §5 ("The nvm Special Case"), three strategies exist.
This PR uses a hybrid:

Method	Strategy	Subprocess?
Detect()	File existence check on ~/.nvm/nvm.sh (or $NVM_DIR/nvm.sh)	No
ListInstalled()	Read <nvmDir>/versions/node/* directory entries (Strategy C)	No
Version()	source nvm.sh && nvm --version via platform.RunShellScript (Strategy A)	Yes

The plan itself recommends Strategy C for reads ("most reliable for
listing/detecting versions") and Strategy A only for mutating
operations. We follow that.

Test seams

Two package-level vars, swappable per-test, mirroring the FNM
pattern from PR #1:

var listDir   = os.ReadDir                  // Strategy C: filesystem read
var runScript = platform.RunShellScript     // Strategy A: shell invocation

Plus a withStubNVMScript helper that writes a real (empty) nvm.sh
file to a temp dir for Detect() tests, sidestepping the complexity
of stubbing os.Stat.

23 tests cover:

• parseNVMVersion × 5 (bare, prefixed, whitespace, empty, whitespace-only)
• parseNVMInstalledEntries × 7 (standard layout, bare versions,
  skips system, skips non-dirs, skips unparseable, empty, only-system)
• NVM.Name, NVM.Detect (2 paths), NVM.Version (3 paths),
  NVM.ListInstalled (4 paths), mutation stubs, GlobalNpmPrefix

Live verification

Probe (one-off dev binary, since deleted) against a real
~/.nvm install:

Detect:        true
Version:       0.40.5
Installed:     3 versions: [16.19.0 18.14.0 19.6.0]

The actual ~/.nvm/versions/node/ contains v16.19.0, v18.14.0,
v19.6.0. Output matches.

Out of scope (intentional)

Mutation methods (Install, Uninstall, Use, SetDefault,
GlobalNpmPrefix) return ErrNVMNotImplemented, same as FNM. They
will be filled in when the upgrade engine (Phase 2) needs them.

Test plan

• go vet ./internal/detector/... — clean
• go test -race ./internal/detector/... — 23 NVM tests pass,
  full package (FNM + registry + NVM) green
• go build ./... — repo-wide build clean
• go test -race ./... — all packages green
• Live probe — confirmed against real ~/.nvm
• CI on this PR — please watch the green ✓

[cocogitto-bot]

✔️ 043e100 - Conventional commits check succeeded.