Docs/output format flag position

[arcobaleno64]

Summary

• Document the confirmed invocation order for the global --output-format option.
• Add machine-readable output examples for the models and agents subcommands.
• Make the existing functionality easier to discover from the README.

Context

Issue #777 originally reported that models and agents rejected the --output-format option when it was placed after the subcommand.

A maintainer subsequently provided the working invocation order:

agy --output-format json models

The issue was later re-tested with AGY 1.1.15, confirming that placing the global option before the subcommand produced machine-readable output for models and agents.

This documentation-only change records that confirmed invocation pattern. It does not change command-line parsing or make compatibility guarantees about other argument orderings or future releases.

Changes

The README now includes these examples:

agy --output-format json models
agy --output-format stream-json agents

Validation

• Ran git diff --check.
• Confirmed that only README.md is modified.
• No executable code or command-line behavior is changed.

Addresses #777.