v1.15.5 - Fix Pi exitCode handling

Fixed

• Pi Custom Tools: Fix exitCode handling for successful commands
  • Changed from if (result.exitCode !== 0) to if (result.exitCode && result.exitCode !== 0)
  • Handles Pi's exec() returning undefined/null/0 for successful commands
  • Prevents false positive failures when tools return correct data
  • Affected tools: invar_sig, invar_map, invar_doc_toc, invar_doc_read, invar_doc_find, invar_doc_replace, invar_doc_insert, invar_doc_delete

This completes the three-part fix for Pi custom tools:

• v1.15.3: Parameter validation and defaults
• v1.15.4: Error message capture from stdout/stderr
• v1.15.5: Exit code handling (this release)