Define stable structured-log, JSON output, and JSON error contracts

[codeforester]

Parent: #50

Train position: 13 of 22
Phase: P2 — Operational completeness

Problem

Logging is human-formatted only, and machine consumers lack stable versioned success and error envelopes.

Acceptance criteria

• Opt-in JSON logs have a versioned schema including timestamp, level, logger, message, and run_id.
• Success and failure use documented versioned JSON envelopes with stable code, type, message, details, and run_id fields.
• Exit mappings are deterministic; machine output never mixes human prose or ANSI on stdout.
• Sensitive fields remain redacted and persistent logging has bounded, privacy-conscious defaults.
• Human defaults remain compatible, with schema and contract tests.

Dependencies

• Finalize run metadata and normalize every invocation outcome #52
• Make standard lifecycle options independently composable #58
• Publish stable typed runtime, profile, and context extension contracts #59