Add a realistic preview mock filesystem and file RPC support (#3042)

The preview environment had no useful filesystem model, which limited
FileSystem integration work and made file-oriented previews hard to
exercise. This change adds a realistic mock filesystem rooted at
`/Users/mike` and teaches `mockwaveenv` to answer the core file RPCs
used by preview flows.

- **Mock filesystem dataset**
  - Adds a dedicated mock filesystem module for preview usage.
- Seeds a macOS-style tree rooted at `/Users/mike` with 100+ file
entries across common directories like `Documents`, `Downloads`,
`Pictures`, `Projects`, and `waveterm`.
- Includes a small set of files with actual payloads so preview
consumers can render meaningful content:
    - text (`.bashrc`)
    - markdown (`meeting-notes.md`, repo docs)
    - images (`.png`, `.jpg`)

- **Mock file RPC implementations**
  - Wires preview-backed implementations for:
    - `FileInfoCommand`
    - `FileReadCommand`
    - `FileListCommand`
    - `FileJoinCommand`
    - `FileListStreamCommand`
    - `FileReadStreamCommand`
  - Supports:
    - `wsh://...` URIs and plain paths
    - path normalization (`~`, relative paths, `.` / `..`)
    - directory reads returning `entries`
    - hidden file filtering via `FileListOpts.all`
    - offset/size range reads
    - streamed directory batches and file chunks

- **Preview behavior improvements**
  - Makes directory previews behave more like a real local filesystem.
- Makes markdown-relative asset resolution via `FileJoinCommand` work
against a coherent tree.
- Gives future FileSystem integration work a stable preview substrate
instead of ad hoc RPC stubs.

- **Focused coverage**
  - Adds preview mock tests for:
    - filesystem population shape
    - file metadata lookup
    - file reads
    - directory listing
    - path joining
    - stream responses

Example:

```ts
const joined = await env.rpc.FileJoinCommand(null as any, [
    "wsh://local//Users/mike/Documents",
    "../waveterm/docs",
    "preview-notes.md",
]);

const file = await env.rpc.FileReadCommand(null as any, {
    info: { path: "/Users/mike/.bashrc" },
});
```

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>
Co-authored-by: sawka <mike@commandline.dev>

Author: Copilot