Dot-relative chat file links resolve from filesystem root

## Symptom

In Berd 0.6.2, inside the main window with a valid attached session folder, a Markdown link such as `[Open](./report.html)` fails with:

```text
File not found: /report.html (session cwd: /Users/.../project)
```

The same existing file opens when the link destination is written as `report.html`. External HTTPS links also open normally.

## Cause

The Markdown hardening pipeline rewrites `./report.html` and `../report.html` to `/report.html` before the artifact click handler resolves the destination against the session working directory.

This is separate from #166, which covers opener permissions outside `$HOME`.

## Acceptance

- Preserve safe `./` and `../` Markdown destinations exactly through rendering.
- Continue blocking unsafe schemes and forged internal sentinels.
- Cover both dot-relative forms with renderer-level regression tests.

Start by locating the Markdown hardening pipeline and the artifact click handler in the renderer, then inspect the existing renderer-level tests. Reproduce the handling of ./report.html and ../report.html with an attached session folder. Done means both dot-relative destinations remain usable while unsafe schemes and forged internal sentinels remain blocked.

Written by the indexing model from the issue text.