Add reference multi-repo examples

[codeforester]

Summary

• Add read-only reference examples for mani, gita, vcs2l, and west under examples/tooling/multi-repo/.
• Align those examples with workspace.yaml.example while keeping Base discovery authoritative.
• Add contract and validation guards so external multi-repo formats do not become active root-level synchronization state.

Fixes #181.

Validation

• ./tests/validate.sh
• git diff --check