fix: preserve existing GitHub secrets during identity setup

[Alexander Zaslonov (azaslonov)]

Summary

• inventory repository and environment secret names before identity setup writes
• require explicit overwrite, scope-aware rename, or reuse decisions for conflicts
• keep inventory and writes pinned to the selected repository and validate generated environment names
• update workflow secret references safely when environment mappings differ
• add regression coverage for target parity, collisions, workflow mappings, and unsafe environments

Validation

• npm run build
• npm run lint
• npm test (1,116 tests)
• constitution review: approved with no findings

Related Issue(s)

Closes #121