fix(code): trust Hub host for code-server links

[MioYuuIH]

Summary

• Automatically derive the current public Hub host and inject it as a code-server trusted domain for Back-to-Hub links.
• Add optional custom.codeServer.extraTrustedDomains values for deployment-specific trusted sites without hardcoding platform domains.
• Translate trusted domains into code-server --link-protection-trusted-domains flags and document the runtime contract.

Verification

• python3 -m pytest runtime/hub/tests/test_git_clone_persistence.py runtime/hub/tests/test_config_resource_metadata.py
• bash -n dockerfiles/Code/start-code-server.sh
• python3 scripts/generate_values_schema.py