[Bug]: Opening a context-less Solo file leaks and re-saves the previous model context

## Summary

Opening a context-less model in Precogly Solo retains the System Context from the previously opened model. Saving the new model then writes that previous model's context into the new file.

## Steps to reproduce

1. Open File A in Solo. File A contains a distinctive system description, for example `File A confidential payment context`.
2. Open File B. File B is a valid CycloneDX model whose Precogly visualization has no `systemContext`.
3. Open **Add / Edit Context → System**.
4. Save File B and inspect its downloaded CycloneDX JSON.

## Actual behavior

- File B's System panel displays `File A confidential payment context`.
- Saving File B writes that value to:

```text
blueprints[0].visualizations[0].data.systemContext.systemInfo.description
```

No warning indicates that state from another document was retained or added.

## Expected behavior

Opening File B should fully replace document-scoped state. If File B has no System Context, the context form and exported `systemContext` should use a clean default rather than retain File A's values.

## Browser reproduction against current `main`

Reproduced in headless Chrome against commit [`4f429a8`](https://github.com/precogly/precogly/commit/4f429a8d4e7c9b0192cf5b6cd0826db702bbebf4) with two in-memory CycloneDX files.

The focused Playwright journey verifies File A's description, opens context-less File B, saves B, reads the downloaded JSON, and fails with:

```text
Expected: ""
Received: "File A confidential payment context"
```

## Cause indicated by the current implementation

The layout loads context only when the newly opened file supplies a truthy `systemContext`; there is no reset branch:

https://github.com/precogly/precogly/blob/4f429a8d4e7c9b0192cf5b6cd0826db702bbebf4/frontend/src/features/guest-editor/GuestLayout.tsx#L194-L203

The header repeats the same conditional behavior when replacing threat-editor state:

https://github.com/precogly/precogly/blob/4f429a8d4e7c9b0192cf5b6cd0826db702bbebf4/frontend/src/features/guest-editor/components/GuestEditorHeader.tsx#L265-L278

Save always retrieves and serializes the currently held context, including the retained File A values:

https://github.com/precogly/precogly/blob/4f429a8d4e7c9b0192cf5b6cd0826db702bbebf4/frontend/src/features/guest-editor/components/GuestEditorHeader.tsx#L161-L175

## Impact

This silently contaminates one threat model with another model's assumptions and system description. Besides producing incorrect analysis evidence, it can disclose sensitive architecture context if File B is later shared with a different audience.

## Relationship to existing issues

- #477 covers warning about unsaved edits before opening another file, not clearing document-scoped state after the open.
- Closed #186 introduced System Context in Solo.
- Closed #508 concerns fields from the current imported file being dropped, not values leaking in from a previous file.

No existing issue found covers this cross-file context retention and re-export path.

Start in frontend/src/features/guest-editor/GuestLayout.tsx and GuestEditorHeader.tsx at the linked context-loading and save paths. Run the focused Playwright journey described in the issue, then inspect the state transition when opening a model without systemContext. Done means File B shows a clean context and its downloaded CycloneDX JSON does not contain File A's description.

Written by the indexing model from the issue text.