fix: fixed vcs

[separatrixxx]

When a TOC is included with mode: merge, files are physically copied to a new location. TocService recorded the original file path as sourcePath in the merged file's metadata

This caused two bugs:

1. Missing repo prefix. sourcePath was set to a project-relative path instead of the full VCS-relative path
2. Explicit vcsPath overridden. If the original file had an explicit vcsPath in its frontmatter, the TocService fallback still wrote sourcePath = from=

Instead of storing from as sourcePath in the merged file's metadata, store it in a dedicated mergeFrom field

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud