docs: slim AGENTS.md to defer to the developer guide (#538)

The file duplicated API reference, testing, patterns, and IaC content
that the AWS Durable Execution SDK Developer Guide now owns. Duplicated
examples drift from the released API. The wait_for_callback examples
showed a one-argument submitter, which raises TypeError against the
current SDK, and imported WaitForCallbackConfig from the wrong module.

Replace the duplicated content with links to the developer guide and
keep only what is repo-specific: the replay-model rules in prose, the
package layout, and the hatch-based contribution workflow.

Author: yaythomas