Define versioned JSON success and error envelopes

[delaudio]

Parent epic: #31

Problem

Machine consumers need a stable contract that is independent of terminal formatting, colors, and prose changes.

Scope

Define the common protocol used by every structured Hum command.

A successful response should expose at least:

• schemaVersion
• command identity
• project and template context when applicable
• a command-specific data payload

A structured failure should expose at least:

• schemaVersion
• a stable error code
• a safe human-readable message
• optional structured details
• the documented process exit code

Acceptance criteria

• Structured output never contains ANSI control sequences.
• stdout contains only the structured payload in structured mode.
• diagnostics that are not part of the payload go to stderr.
• Error codes and exit-code mappings are documented and tested.
• Serialization is deterministic enough for contract fixtures.
• Secret values, provider output, tokens, and raw environment maps are excluded.
• Existing human-readable commands remain backward compatible.
• Shared Rust types/helpers prevent each command from inventing its own envelope.