chore: purge _build/ from tracking + .gitignore refresh

Removes 102 accidentally-tracked Elixir build artefacts from under
demo/_build/ (compile.lock, .beam, .mix/compile-* files that should
never have been committed). .gitignore updated to prevent recurrence.

Paired with the 2026-04-17 estate move: snif now lives at
/var/mnt/eclipse/repos/snif (top-level, was nested under
verification-ecosystem/ — it's a type-system-backed NIF safety tool,
not a satellite of a verification framework; the old placement was
drift).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -104,3 +104,11 @@ sync_report*.txt
 # Hypatia scan cache (local-only)
 .hypatia/
 .zig-cache/
+target/
+node_modules/
+_build/
+deps/
+.elixir_ls/
+.cache/
+build/
+dist/