Discover legacy backup roots and simplify Documents handling

[gaoguobin]

Motivation

• Ensure backup discovery is robust across older installations that used ~/CodexBackups in addition to ~/Documents/... so list_backups finds legacy backups.
• Stop treating a non-writable ~/Documents as a reason to route backups away from the normal Documents location, because writability checks are unnecessary for discovery.
• Add an explicit validation note to the skill docs to make sandbox/permission testing deterministic by reusing an explicit --backup-root.

Description

• Remove the writability check from default_backup_root so only a non-directory Documents triggers the fallback to the legacy root.
• Add default_backup_roots_for_discovery and update list_backups to search both the primary Documents location and the legacy fallback, deduplicate manifests, handle unreadable manifests, sort results by created_at descending, and expose discovery_roots when discovery was used.
• Update README.md wording to reflect the directory vs writable-directory behavior and add a validation note to skills/claude-code-environment-backup/SKILL.md and skills/codex-environment-backup/SKILL.md about passing a consistent --backup-root during cross-boundary testing.

Testing

• Ran unit tests in tests/test_core.py including new tests test_default_backup_root_ignores_writability_checks, test_list_backups_discovers_legacy_home_fallback_root, and test_list_backups_default_discovery_is_sorted_by_created_at_desc.
• Existing related tests were retained and adjusted for the new behavior.
• All automated tests passed locally.

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Codex Task

[chatgpt-codex-connector]

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