chore(gitignore): ignore dune-generated *.install manifests (#306)

`affinescript.install` (and any sibling `*.install` files for future
opam packages declared in `dune-project`) is regenerated by dune on
every `dune build`.  It was showing up as untracked across recent
sessions, one stray `git add .` away from being committed alongside
the regular source changes.

`*.install` is the standard pattern: dune emits one per `(package ...)`
declaration in `dune-project`, and they're consumed only by opam during
install — not durable repo content.  Same shape as the previous
`target/` glob fix (#296) — match anywhere, not just the repo root, so
any future nested opam package's `.install` file is covered too.

Verified post-fix: `git check-ignore affinescript.install` returns the
path; `git status` no longer lists it as untracked.

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

Author: hyperpolymath

.gitignore

@@ -17,6 +17,8 @@ target/
 /build/
 /dist/
 /out/
+# dune-generated package install manifests (sibling to *.opam — regenerated on build)
+*.install
 # Idris2 build artifacts (nested, e.g. under formal-verification dirs)
 **/solo-core/build/