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/tools/ConfigTool/prompt.ts

@@ -59,7 +59,7 @@ export function generatePrompt(): string {
 ## Configurable settings list
 The following settings are available for you to change:
 
-### Global Settings (stored in ~/.claude.json)
+### Global Settings (stored in ~/.openclaude.json)
 ${globalSettings.join('\n')}
 
 ### Project Settings (stored in settings.json)