Fix validator exit safety and false-success reporting

[AM-979]

Summary

This PR replaces the unsafe validator-exit execution path and adds validation, auditability, documentation, regression tests, and CI.

Root cause

The previous implementation passed a list containing echo, a sudo password, a pipe token, and the Lighthouse command to subprocess.run(..., shell=True). On Linux this could execute only echo, return status 0, and then record a false successful validator exit without invoking Lighthouse.

Changes

• remove sudo password collection, shell pipelines, and shell=True
• invoke Lighthouse with a structured argument list
• require exact EXIT N VALIDATOR(S) confirmation after displaying all selected public keys
• reject zero, invalid, negative, and out-of-range counts
• validate YAML structure, validator public keys, duplicates, keystore files, password files, Lighthouse executable, and custom-network directory
• support Lighthouse password paths and securely handle legacy inline passwords via temporary mode-0600 files
• keep Lighthouse waiting for an exiting/exited state before recording confirmed
• add configurable CLI options and Jibchain defaults
• replace truncating text logs with append-only mode-0600 JSON Lines audit records
• correct withdrawal timing guidance and README/schema examples
• add dry-run and stop-on-error behavior
• add regression tests and GitHub Actions for Python 3.8, 3.10, and 3.12

User impact

The helper no longer reports a successful exit merely because an unrelated shell command returned zero. Users can inspect the exact validators and generated commands before submitting an irreversible operation, and every attempt is preserved in a private audit log.

Validation

GitHub Actions completed successfully on Python 3.8, 3.10, and 3.12. Each matrix job passed dependency installation, Python compilation, and the complete unit-test suite.

The regression suite covers shell-free invocation, false-success prevention, count validation, YAML handling, Web3Signer filtering, temporary password files, Lighthouse failures, dry-run behavior, and append-only private logs.