[State Sync Redesign 2.7] Remove `@metamask/obs-store` dependency

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After `this.store`, `this.memStore`, and `publicConfigStore` are removed, `ComposableObservableStore` and `ObservableStore` have no consumers. Remove the dependency.

## Steps

1. Delete `app/scripts/lib/ComposableObservableStore.js` and its test
2. Remove `@metamask/obs-store` from `package.json`
3. Run `yarn lint:lockfile:dedupe:fix`
4. Run `yarn lavamoat:auto`
5. Run `yarn attributions:generate`

## Files

- `package.json`
- `yarn.lock`
- `lavamoat/`
- `attribution.txt`
- `app/scripts/lib/ComposableObservableStore.js`

## Risk

Low — no consumers remain.

## Dependencies

Issues 2.4, 2.5, Epic 1 Issue 1.1, and Epic 3 Issue 3.9 (Redux `metamask` slice removed, so no consumers of obs-store-derived state via Redux)

## Context

Part of [State Sync Redesign — Epic 2: Background cleanup].

Start with app/scripts/lib/ComposableObservableStore.js and its test, then inspect package.json and the dependency references in yarn.lock, lavamoat/, and attribution.txt. Delete the unused store and test, remove @metamask/obs-store, and run yarn lint:lockfile:dedupe:fix, yarn lavamoat:auto, and yarn attributions:generate; done means the dependency and generated references are removed without consumer regressions.

Written by the indexing model from the issue text.