chore(detector): remove stale fnm not-implemented comment, sentinel.go package-doc collision, dead config.FileOverlay #104

Summary

Three small stale-comment / dead-code residuals found during the 2026-07-03 re-review (the pass that verified fix PRs #77-#101). None affect behavior; all mislead readers. One PR, three commits or one commit — small enough for a single chore PR.

Items + fixes

1. internal/detector/fnm.go:26-49 — stale "not implemented" comment + dead sentinel error

The FNM struct doc still says:

Mutation methods (Install, Uninstall, Use, SetDefault, GlobalNpmPrefix) return an explicit "not implemented" error…

and ErrFNMNotImplemented is still declared. But all five mutation methods have been real implementations since PR #56, and nothing returns or tests that error anymore (grep -rn ErrFNMNotImplemented → declaration only).

Fix: delete ErrFNMNotImplemented; rewrite the struct doc to describe the actual implemented surface (e.g. "Install/Uninstall/Use/SetDefault shell out to fnm <cmd>; GlobalNpmPrefix resolves the per-version prefix dir"). Deleting the exported var is safe — zero references repo-wide.

2. internal/packages/sentinel.go:1 — wrong package name in doc comment

File opens with // Package sentinel implements… but the package is packages. go doc attributes this comment to package packages, so the rendered docs are wrong/confusing.

Fix: demote it to a file-level comment that doesn't collide with godoc convention — e.g. start with "Sentinel file handling: …" (any first word other than "Package"), keeping the existing explanation text. Verify only one file in the package carries the canonical // Package packages … comment.

3. internal/config/resolve.go:93-105 — superseded FileOverlay left behind

FileOverlay(cfg) (marks every field as set, which is exactly the explicit-zero-value bug) was superseded by FileOverlayFromNode (internal/config/fileoverlay.go, used in load.go:48). The old function now has zero call sites outside its own definition, and the comment at resolve.go:71 still points readers at it as "the normal way".

Fix: delete FileOverlay; update the resolve.go:71 comment to reference FileOverlayFromNode. If a coarse all-fields overlay is ever wanted for tests, tests can build an Overlay literal directly.

Acceptance criteria

• grep -rn "ErrFNMNotImplemented\|not implemented" internal/detector/fnm.go → no hits.
• go doc ./internal/packages shows a single, correctly-attributed package comment.
• grep -rn "FileOverlay(" internal/ matches only FileOverlayFromNode.
• make ci green (deletions only — no behavior change, so no new tests needed).