Fix metadata link checker paths

[capaj]

Summary

• use the repository root as the default metadata-check directory
• load social-media patterns from the current src/data/socialmedia.yaml path

Why

The Astro collection migration moved logo metadata to src/content/logos and social-media definitions to src/data, but bin/chkmetalink.py still starts from the removed www tree and looks for www/_data/socialmedia.yaml. As a result, the metadata-link step exits before checking any logo.

Verification

• reproduced the pre-fix exit on missing www/_data/socialmedia.yaml
• python3 bin/chkmetalink.py under the workflow Python 3.11 environment: 10,206 files checked, 0 errors
• invoked the script from outside the repository to verify that its default root remains script-relative
• focused custom-directory fixture: valid link exits 0; invalid link is discovered and exits 1
• python3 -m py_compile bin/chkmetalink.py
• git diff --check

This PR changes only the two stale path expressions. It is independent of all logo submissions.