Markdown wiki connector (GitHub Wiki, GitLab, etc.)

[fuseraft]

Implement the markdown wiki connector described in docs/features/connectors.md under "Planned Connectors".

Planned scope (from docs):

• Wiki page extraction
• Cross-reference resolution
• Image and asset handling
• Sidebar/navigation preservation

Target CLI surface:

compendium connect wiki \
  --url https://github.com/example/repo/wiki \
  --bundle my-catalog \
  --type Document

Depends on the connector plugin architecture issue (IConnector + ConnectorRegistry).