Add root .gitignore (PM_GH_REPO_005)

[anish-sujanani]

Add a root .gitignore

Addresses PM_GH_REPO_005 — All repositories must define a .gitignore file.

This repository has no .gitignore at its root. One new file, 157 lines, nothing else.

What was chosen and why

The language signal for this repository resolves to the upstream Node template.

Templates are taken verbatim from github/gitignore, pinned at commit 356fd7baab4c05e092194a41f64dbd5afc8817e4, with a provenance header per section.

Verification

• Exactly one file in the diff, at the repository root; no nested .gitignore touched.
• Every already-tracked file was checked against the new rules with
  git check-ignore --no-index — no tracked file matches any of them.
• .env.* precedes !.env.example deliberately: git applies the last matching pattern, so
  reversing them would ignore a committed example file too.

Break risk

None at runtime. Adding a .gitignore changes nothing about existing tracked files and
nothing about CI.