chore(ci): gitignore the local bin/ and coverage.out

[dipto0321]

Summary

make build writes ./bin/nodeup and make test writes
coverage.out, but neither path was in .gitignore — so they
showed up as untracked on every git status, cluttering the
view of what's actually changed during a PR.

Both are local artefacts that should never be committed. This
PR adds them to .gitignore with a comment explaining that the
npm wrapper's install.js populates a different bin/ under
nodeup-npm/node_modules/ (handled by nodeup-npm/.npmignore,
not the root gitignore).

Type of change

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

Checklist

• Title follows Conventional Commits (chore(ci): subject)
• I ran make ci locally — .gitignore-only change, no code change
• I added or updated tests — N/A
• I updated relevant docs — N/A
• No new linter warnings

Scope notes / things reviewers may want to look at

• No risk of accidentally ignoring tracked files. git ls-files bin/ and git ls-files coverage.out both return empty — nothing
  tracked is in either path. The PR is safe to merge even if a
  contributor has stale local builds lying around.
• Why a separate PR. The .gitignore cleanup is unrelated to
  the OIDC publish work in chore(ci): publish nodeup-cli to npm via OIDC Trusted Publishing #39 and the npm wrapper rename in chore(release): rename npm wrapper to nodeup-cli #37.
  Keeping it in its own PR makes both the diff and the merge
  history easier to reason about — #39 is a CI change, #40 is
  a dev-hygiene change.

How to verify

# before: untracked bin/ and coverage.out visible
git checkout main && make build && make test
git status   # shows bin/nodeup and coverage.out as untracked

# after merge
git pull
make build && make test
git status   # clean working tree

[cocogitto-bot]

✔️ 208c0da - Conventional commits check succeeded.