docs: add READMEs for all packages, move sync-readme to docs/

packages/json-cas-fs/README.md

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+# @uncaged/json-cas-fs
+
+Filesystem-backed CAS store.
+
+## Overview
+
+`@uncaged/json-cas-fs` implements a persistent `Store` on disk. Each node is stored as `<hash>.bin` (CBOR-encoded `CasNode`). A `_index/` directory maps type hashes to content hashes for `listByType`. Stores support bootstrap via the same `BOOTSTRAP_STORE` symbol as the in-memory implementation.
+
+Depends on `@uncaged/json-cas` for hashing, CBOR encoding, and types.
+
+**Dependencies:** `@uncaged/json-cas`, `cborg`
+
+## Installation
+
+```bash
+bun add @uncaged/json-cas-fs
+```
+
+## API
+
+Exported from `src/index.ts`:
+
+```typescript
+function createFsStore(dir: string): BootstrapCapableStore;
+```
+
+`BootstrapCapableStore` is re-exported from `@uncaged/json-cas` (via the return type). The store loads existing `.bin` files on open and migrates or builds the type index on first use.
+
+### Example
+
+```typescript
+import { bootstrap, putSchema } from "@uncaged/json-cas";
+import { createFsStore } from "@uncaged/json-cas-fs";
+
+const store = createFsStore("./my-cas-store");
+await bootstrap(store);
+
+const typeHash = await putSchema(store, {
+  type: "object",
+  properties: { id: { type: "string" } },
+  required: ["id"],
+  additionalProperties: false,
+});
+
+const hash = await store.put(typeHash, { id: "item-1" });
+console.log(store.has(hash)); // true after restart if same dir
+```
+
+### On-disk layout
+
+```
+my-cas-store/
+├── <hash>.bin          # CBOR CasNode
+├── _index/
+│   └── <typeHash>      # newline-separated content hashes
+└── ...
+```
+
+Writes use atomic rename (`<hash>.tmp` → `<hash>.bin`).
+
+## Internal Structure
+
+| File | Purpose |
+|------|---------|
+| `store.ts` | `createFsStore`, load/save nodes and type index |
+| `index.ts` | Public export |
+| `store.test.ts` | Filesystem store tests |