CompositeFilesystem

Routes file operations to different AbstractFilesystem stores by path prefix.

Paths are matched against route prefixes (longest first). Unmatched paths fall through to the default store.

Composite deliberately implements only AbstractFilesystem — it is the unified, non-sandbox view that blends a local workspace with remote-store-backed paths. Shell execution is intentionally not supported in this mode: routing shell commands across stores is ambiguous, and the primary use case (distributed memory with per-user/session isolation) does not need it. If you need shell execution, use a sandbox-backed filesystem (AbstractSandboxFilesystem) or LocalFilesystemWithShell directly instead.

Example:

`CompositeFilesystem fs = new CompositeFilesystem(
localFs,
Map.of("/memories/", storeFs, "/cache/", inMemoryFs)
);
fs.read(RuntimeContext.empty(), "/memories/notes.md", 0, 100); // → storeFs.read(...)
fs.read(RuntimeContext.empty(), "/src/Main.java", 0, 100); // → localFs.read(...)
`