docs: cover defensive JSON parsing, empty-output retries, and resume caching (#138)

agent() always resolves to raw text unless `schema` is set, and a call
whose only attempts ended in a recoverable failure (e.g. AGENT_EMPTY_OUTPUT)
is never journaled. Document both facts in README and the workflow-authoring
skill, add a defensive-json-parsing example alongside structured-output.js,
and note agentRetries: 1-2 as fleet guidance for models with occasional
empty output.

Refs #134, #135