fix: rename .claude.json to .openclaude.json with legacy fallback (#582)

* fix: rename .claude.json to .openclaude.json with legacy fallback

Rename the global config file from ~/.claude.json to ~/.openclaude.json,
following the same migration pattern as the config directory
(~/.claude → ~/.openclaude).

- getGlobalClaudeFile() now prefers .openclaude.json; falls back to
  .claude.json only if the legacy file exists and the new one does not
- Add .openclaude.json to filesystem permissions allowlist (keep
  .claude.json for legacy file protection)
- Update all comment/string references from ~/.claude.json to
  ~/.openclaude.json across 12 files

New installs get .openclaude.json from the start. Existing users
continue using .claude.json until they rename it (or a future explicit
migration).

* test: add unit tests for getGlobalClaudeFile migration branches

Covers the three cases:
- new install (neither file exists) → .openclaude.json
- existing user (only legacy .claude.json exists) → .claude.json
- migrated user (both files exist) → .openclaude.json

---------

Co-authored-by: Zartris <14197299+Zartris@users.noreply.github.com>

src/migrations/resetAutoModeOptInForDefaultOffer.ts

@@ -12,7 +12,7 @@ import {
  * One-shot migration: clear skipAutoPermissionPrompt for users who accepted
  * the old 2-option AutoModeOptInDialog but don't have auto as their default.
  * Re-surfaces the dialog so they see the new "make it my default mode" option.
- * Guard lives in GlobalConfig (~/.claude.json), not settings.json, so it
+ * Guard lives in GlobalConfig (~/.openclaude.json), not settings.json, so it
  * survives settings resets and doesn't re-arm itself.
  *
  * Only runs when tengu_auto_mode_config.enabled === 'enabled'. For 'opt-in'