Porcelain fsck - verify repository integrity

## Summary
Implement `fsck` command that calls plumbing `fsck` and displays issues.

## Context
User-facing integrity check.

## Tasks
- Create `porcelain/src/admin/fsck.rs`.
- Call plumbing `fsck`.
- Format errors/warnings.
- Document.

## Acceptance Criteria
- Reports corruption or success.

## Dependencies
- Plumbing `fsck`.
