docs: note what the archive tag is for

Records why archive/feat-bom-generator exists and what was in it, so a future
reader does not have to reconstruct the reasoning from the branch name. Also
notes that archive tags need an explicit push.

ARCHIVE.md

@@ -0,0 +1,11 @@
+# Archive tags
+
+Tags whose only purpose is to keep a commit reachable after the branch that
+carried it was deleted. Nothing here is active code.
+
+| Tag | Kept because |
+| --- | --- |
+| `archive/feat-bom-generator` | The branch held one commit (`8f860e0`, "BOM generator v2 with hashes") that never landed on `master`. The capability — a content hash per asset — was ported forward to `scripts/bom_gen.py --hash` in `2c6e227`, and the TypeScript file it patched (`scripts/bom-gen.ts`) was deleted in `afe397d`. The tag remains so the original commit is reachable if the port is ever questioned. |
+
+Tags are pushed explicitly (`git push origin archive/<name>`); they are not
+picked up by a plain `git push`.