[Cross-cutting] Standardized exit codes across all apps

[markcallen]

Overview

Standardize exit codes across dev-cache, git-cleaner, and mac-cache-cleaner so scripts can reliably detect success, failure, and "nothing to do" states.

Implementation Plan

1. Convention

   Code	Meaning
   0	Success
   1	General error (config, flags, runtime)
   2	Nothing to clean / no findings (optional)
   3	Partial failure (some operations failed)

2. Per-app behavior

   • dev-cache: 0 = success; 1 = config/init error; 2 = no findings when that's distinct
   • git-cleaner: Same
   • mac-cache-cleaner: Same; 3 = some targets failed

3. Implementation

   • Replace os.Exit(1) with consistent codes
   • Document in README and --help
   • Scripts can branch: if [ $? -eq 2 ]; then echo "Nothing to clean"; fi

Acceptance Criteria

• All apps use exit code 0 for success, 1 for error
• Optionally 2 for "nothing to do", 3 for partial failure
• Documented in README