Stabilise the metadata sync workflow and add offline validation

Restructure the script embedded in the scoped sync workflow into testable
functions, fix the invalid `exit 0` (now a successful run with a visible
warning when Gitea returns an invalid push mirror list) and report errors as
`ERROR: <message>` with exit code 1. Zero, several or malformed GitHub mirrors
still fail.

Add pytest-based offline tests (workflow YAML, embedded Python, credential and
mirror parsing, sync flow against a fake API) that run in a virtual
environment, and a validation workflow that runs them on pull requests and
pushes to main. Remove the manual tests/test_.ps1 probe and the duplicate
src/ copy of the workflow. Update the README and record the decisions for
tasks 1 and 3 in MIL-001.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Task: MIL-001#6
Refs #1
Refs #2
Refs #3
Refs #4
Refs #5
Refs #6

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/requirements.txt

@@ -0,0 +1,2 @@
+pytest>=8,<10
+PyYAML>=6,<7