fix(deepagents): accept permission paths equal to a composite route root (#768)

## Description

When filesystem `permissions` are combined with an execution-capable
backend, `createFilesystemMiddleware` requires every permission path to
be scoped to a `CompositeBackend` route. That check required a trailing
separator, so a permission path that exactly equals a single-file mount
(route `/instructions.md`, permission `/instructions.md`) was rejected
and users had to write `/instructions.md/**`. Python deepagents accepts
the exact form; this syncs JS with it by also accepting the route root
itself, mirroring `CompositeBackend.isPathWithinRoute`. It deliberately
does not adopt Python's looser bare `startsWith`, so sibling prefixes
like `/workspace2/**` for route `/workspace` are still rejected.

## Test Plan
- [ ] `npx vitest run src/middleware/fs.permissions.test.ts` in
`libs/deepagents` (new cases: exact file-mount route and exact directory
route root are accepted; sibling-prefix and `/**` rejections unchanged).

Made by [Open
SWE](https://openswe.vercel.app/agents/2758c734-16e9-54c3-b89f-ce698a566e71)

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Co-authored-by: Caspar Broekhuizen <25157475+casparb@users.noreply.github.com>
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Hunter Lovell <hunter@hntrl.io>