Add gitleaks pre-commit hook and missing README endpoints

[splch]

Summary

Follow-up to #15. Addresses items 6-10 from the open-source readiness audit:

• Add gitleaks to pre-commit hooks (item 6) - Industry standard secret scanning per OpenSSF Best Practices Badge criteria. Catches secrets before push; GitHub secret scanning (enabled in Add contributor license section and PR template #15) is the backstop.
• Enable CodeQL default setup (item 7) - Enabled via repo API (Python language). Satisfies OpenSSF Scorecard SAST check.
• Python version floor (item 8) - Keeping >=3.12 as a deliberate choice. Already documented in README.
• Add missing endpoints to README table (item 9) - Added get_job_probabilities, get_variant_histogram, get_variant_probabilities, get_variant_shots.
• Overlay file references (item 10) - No changes needed. The openapi-overlay.yaml is intentionally absent when the upstream spec matches and the existing docs already explain this.

Repo settings updated separately (not in this diff):

• CodeQL default setup enabled for Python (run started: https://api.github.com/repos/ionq/ionq-core-python/actions/runs/24901094251)
• Code Security enabled (prerequisite for CodeQL)

Test plan

• .pre-commit-config.yaml is valid YAML with correct gitleaks rev
• README endpoint table now lists all modules in ionq_core/api/default/
• No changes to source code or tests