Allow sub-path exclusions in `[identity] produces`

In React Native / Expo projects, native build output is written into node_modules and dwarfs the dependencies:

A tree that starts at 1.3 G grows to ~9 G after expo run:android, and ~85% of that is derived output no other instance wants. The reference is upfront that copy-on-write forks on write and explicitly cites native builds as the reason it isn't hardlinks — that is the right call and it works. But it means the ecosystem where installs are most expensive is the one where the store helps least after the first native build.

An exclusion list would keep the stored entry close to the install:

[identity]
produces = ["node_modules"]
produces_exclude = ["node_modules/*/android/.cxx", "node_modules/*/android/build"]

This fights a stated rule — a prepare must be the tree's whole output — and the same logic says a partial tree isn't a faithful entry. So this may well be a "no, and here's why." Filing because the ecosystem impact is large and the tension seems worth recording either way.

Measured context: with a clean tree, a third instance cost 163 MB marginal disk against 1.4 GB apparent, so the store works exactly as advertised right up until a native build runs.

Environment: newgit 0.3.0 (fba8fa7090e2), macOS 15 (Darwin 25.6.0), APFS. npm-workspaces monorepo (Expo/React Native + Next.js + Supabase); local stack is Supabase CLI 2.107.0 (12 containers, Docker Desktop 28.2.2), Metro, and a Dockerised routing service. 4 instances spawned over one session; resources deps (install), expo (process), supabase (command-snapshot-migrations), r5 (project-owned).

The issue names the [identity] produces configuration and proposes produces_exclude patterns for node_modules native-build output. Start by locating the configuration handling for [identity] produces, then determine whether excluded subtrees can be supported without violating the whole-output rule. Done means either implementing and validating the exclusion semantics or documenting why partial trees cannot be supported.