CAMEL-23823: add AGENTS.md to guide AI agents contributing examples

Adds an AGENTS.md with guidance for AI agents working in this repository:
the shared Apache Camel rules of engagement (attribution, JIRA ticket
ownership, one-example-per-PR, fork-based branches, green CI) plus the
specifics of this repo - the Maven module layout and root pom registration,
Spring Boot starter usage, @CamelSpringBootTest tests, conventions and a
checklist for adding a new example. Complements the canonical apache/camel
AGENTS.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>