Rename and compatibility-plan claude-grc paths, caches, config, and env vars

[ethanolivertroy]

Context

The source docs and connector guidance use Claude-specific local paths and names, including examples such as:

• ~/.local/share/claude-grc/tools/...
• ~/.config/claude-grc/connectors/...
• ~/.cache/claude-grc/findings/...
• CLAUDE_GRC_SCF_CACHE

For a Codex fork, these names are confusing and may conflict with existing Claude installs. However, breaking existing scripts abruptly would be inefficient.

Tasks

• Inventory all claude-grc path/env var references.
• Propose Codex-native equivalents, e.g. codex-grc paths and CODEX_GRC_* env vars.
• Decide backward compatibility policy:
  • support old paths as fallback
  • migration warning
  • cutoff timeline
• Update docs and scripts accordingly.
• Add tests for fallback behavior where scripts read/write config/cache.

Acceptance Criteria

• Codex docs use Codex-native paths.
• Existing users have a clear migration path from Claude-named config/cache locations.