Replace schema command output with runtime JSON schema contract

[ggonzalez94]

Summary

Replace defi schema [command path] output with runtime output JSON schema (envelope + command data shape), even though this is a breaking change for current schema consumers.

Problem

Current schema output describes command/flag metadata, not the response contract. Agents and typed clients still need to infer runtime output shapes manually.

Proposal

• Make defi schema [command path] return JSON Schema for that command's runtime output.
• Include:
  • full envelope schema (version/success/data/error/warnings/meta)
  • data item schema for the selected command
  • explicit optional fields for provider-specific extras (for example execution IDs / fee breakdown fields)
• Keep deterministic field ordering expectations documented separately.

Breaking-change note

This intentionally changes the shape returned by defi schema.

Acceptance criteria

1. defi schema yield opportunities returns JSON Schema (Draft 2020-12) for the output contract, not Cobra command metadata.
2. Schema includes both success and error envelope shapes.
3. Schema covers provider-optional fields already in contract (e.g. provider_native_id, market_address, fee_breakdown).
4. Tests validate schema generation for at least one command in each major domain (lend, yield, bridge, swap).
5. README/AGENTS updated to describe the new schema behavior and breaking change.

Implementation notes

Prefer generating schema from model types + command response wrappers to minimize maintenance drift.