Normalize runtime ledger timestamps at write time

[joeszilagyi]

Fixes #742.

Summary:

• Normalize runtime-ledger timestamps when building events.
• Normalize and validate timestamps again at append time so invalid write-time timestamps are rejected before they hit JSONL.
• Add regressions covering normalized UTC output and invalid timestamp rejection.

Validation:

• python3 -m py_compile tools/common/runtime_ledger.py tests/test_scheduler_failure_state_reconciliation.py
• /tmp/codex-ruff-venv/bin/ruff format --check tools/common/runtime_ledger.py tests/test_scheduler_failure_state_reconciliation.py
• /tmp/codex-ruff-venv/bin/ruff check tools/common/runtime_ledger.py tests/test_scheduler_failure_state_reconciliation.py
• python3 -m pytest -q tests/test_scheduler_failure_state_reconciliation.py