[Bug]: Internal Markdown links without # fragments open edit mode instead of preserving preview

## Operating system

macOS

## Orca version

1.4.188

## Details

### Short summary

Following a relative link from a rendered Markdown preview opens different destination modes depending only on whether the link contains a `#heading` fragment:

- `[Target](target.md)` opens the destination in **edit mode**.
- `[Target](target.md#heading)` opens the destination in **Markdown preview**.

The fragment should control scroll position, not whether navigation leaves the rendered reading surface. The current behavior is surprising and breaks the reading rhythm between linked documents.

### Reproduction

1. Create `source.md`:

```md
# Source

[Plain target](target.md)
[Anchored target](target.md#target)
```

2. Create `target.md`:

```md
# Target

Destination content.
```

3. Open `source.md` using **Open Markdown Preview**.
4. Click **Plain target**.
5. Observe that `target.md` opens in edit mode.
6. Return to the preview and click **Anchored target**.
7. Observe that the same file opens in rendered Markdown preview.

### Expected

Both links preserve Markdown preview. If a fragment is present, the preview additionally scrolls to that heading.

### Actual

The plain link opens an editor tab; only the anchored link opens a preview tab.

### Additional evidence

The packaged 1.4.188 renderer's Markdown document navigation branches on the presence of `options.anchor`: the anchored branch calls `openMarkdownPreview(...)`, while the no-anchor branch calls `openFile(..., mode: "edit")`.

This became particularly confusing with a long Markdown destination containing supported preview content but raw HTML that rich editing would not round-trip: the plain-link route landed in source editing, while manually choosing **Open Markdown Preview** rendered it correctly. That HTML fallback is separate; the core issue reproduces with the two minimal files above.

Start at the Markdown document navigation branch described in the report, where options.anchor selects openMarkdownPreview(...) and the no-anchor path calls openFile(..., mode: "edit"). Reproduce the behavior with the source.md and target.md files from the issue; done means both relative links preserve Markdown preview, with anchored links still scrolling to the heading.

Written by the indexing model from the issue text.