feat: use OS-preferred config directory

[jy-tan]

Summary

Migrate default config path from ~/.fence.json to OS-preferred locations (~/.config/fence/fence.json on Linux, ~/Library/Application Support/fence/fence.json on macOS) while maintaining backward compatibility with the legacy path.

Resolves #25.

Changes

• Config path resolution: DefaultConfigPath() now follows XDG conventions (Linux) and uses ~/Library/Application Support (macOS), with automatic fallback to ~/.fence.json if only the legacy file exists
• Import command UX:
  • Add --save flag to write directly to the default config path
  • Add overwrite confirmation prompt when target file exists
  • Make --save and -o mutually exclusive
  • Preview mode (no flags) prints JSON to stdout without modifying files
• Documentation: Update all docs and help text to reflect the new config paths

Migration Path

Existing users with ~/.fence.json will continue to work automatically. New users will get the XDG-compliant path. Users can explicitly migrate by moving their config to the new location.

[cubic-dev-ai]

1 issue found across 8 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="docs/library.md">

<violation number="1" location="docs/library.md:86">
P2: The example hard-codes a Linux path with `~`, but `LoadConfig` doesn’t expand `~` and macOS uses a different default. This makes the sample misleading and non-portable. Use `fence.DefaultConfigPath()` (or expand `os.UserHomeDir`) in the example instead.</violation>
</file>

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