Add AGENTS.md with security-model link for agent discoverability

[potiuk]

This is a proposal for the PMC to review — please correct,
reject, or discuss as needed. Nothing here is a requirement;
the maintainer is the decision-maker.

This adds an AGENTS.md to apache/airflow-client-python so
an automated scan agent can mechanically discover the project's
security model via the conventional AGENTS.md → model chain.
The client library currently has no in-repo security-discovery
file, so an agent has no way to find the authoritative Airflow
security model without out-of-band knowledge.

Context: the ASF Security team is preparing the project for an
automated agentic security scan we're piloting. Such scans
refuse to run if the model isn't discoverable by that path
(refusing upfront beats wasting PMC reviewer cycles on a
noise-heavy run against an unknown model). Discoverability is
the one hard gate; everything else is suggestion. The Security
team has reached out separately on the PMC's private list with
the program details; this PR is the public-facing repo piece.

The pointer chains the client to the main Apache Airflow
security model (the same one apache/airflow's AGENTS.md
points at). The Security team uses
threat-model-producer
as the rubric for what a complete model looks like — but this
PR is just the link; nothing about the model content itself
changes.

Questions / pushback welcome. Happy to adjust the wording or
move the section if the project has a house style.

🤖 Generated with Claude Code

[potiuk]

cc: @shahar1