feat(persist): add --list-backups and --restore options for backup management

Add backup management functionality to the persist command:
- List available backups with timestamps and dates
- Restore from most recent or specific backup
- Improved error handling for corrupted settings.json
- Updated CLAUDE.md to document the persist command and the exception to the non-invasive constraint

CLAUDE.md

@@ -98,7 +98,7 @@ bun run validate            # Step 3: Final check (must pass)
 
 1. **NO EMOJIS** - ASCII only: [OK], [!], [X], [i]
 2. **TTY-aware colors** - Respect NO_COLOR env var
-3. **Non-invasive** - NEVER modify `~/.claude/settings.json`
+3. **Non-invasive** - NEVER modify `~/.claude/settings.json` without explicit user request and confirmation (exception: `ccs persist` command)
 4. **Cross-platform parity** - bash/PowerShell/Node.js must behave identically
 5. **CLI documentation** - ALL CLI changes MUST update respective `--help` handler (see table below)
 6. **Idempotent** - All install operations safe to run multiple times
@@ -116,6 +116,7 @@ bun run validate            # Step 3: Final check (must pass)
 | `ccs copilot --help` | `src/commands/copilot-command.ts` → `handleHelp()` |
 | `ccs doctor --help` | `src/commands/doctor-command.ts` → `showHelp()` |
 | `ccs migrate --help` | `src/commands/migrate-command.ts` → `printMigrateHelp()` |
+| `ccs persist --help` | `src/commands/persist-command.ts` → `showHelp()` |
 | `ccs setup --help` | `src/commands/setup-command.ts` → `showHelp()` |
 
 **Note:** `lib/ccs` and `lib/ccs.ps1` are bootstrap wrappers only—they delegate to Node.js and contain no help text.