Dot-relative chat file links resolve from filesystem root

[AnobleSCM]

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:

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.

[AnobleSCM]

I reproduced this in the live main window and built a minimal patch locally. Per the repository's outside-contribution policy, I am leaving the implementation guidance here rather than opening a PR.

Narrow implementation shape:

• For href destinations only, normalize a validated ./file destination to its already-supported bare equivalent (file) before the existing local-path sentinel passes through rehype-harden.
• Leave ../ destinations and src handling unchanged.
• Keep the existing bare-path predicate, unsafe-scheme checks, and forged-sentinel behavior unchanged.

Regression coverage:

• Renderer test: [Report](./report.html) produces href="report.html", not /report.html.
• Click integration test: the delegated artifact handler receives report.html and opens the resolved project path.
• Existing resolver tests prove bare paths resolve against the session cwd.

Local verification on current origin/main (5c6a69d8):

• just check passed.
• just test passed: 575 test files, 6,777 tests passed, 1 skipped.
• Focused renderer/resolver/click suite passed: 44 tests.