Add AI_POLICY.md

[mshustov]

Adds the AI usage policy from the main ClickHouse repository: https://github.com/ClickHouse/ClickHouse/blob/master/AI_POLICY.md

This makes the policy consistent across ClickHouse client/driver repositories.

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Note

Low Risk
Documentation-only addition with no runtime, security, or build impact.

Overview
Adds AI_POLICY.md, aligning this repo with the main ClickHouse project’s AI usage guidelines.

The document states that AI is an acceptable development tool (no disclosure required), that contributors remain responsible for reviewing generated code and avoiding low-effort PRs, and that IP/licensing rules apply the same as for hand-written code. It also notes preferred uses (refactors, CI, security, bug fixes) and invites AI research contact at ai@clickhouse.com.

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[github-actions]

Repository collaborators can run the JMH benchmark suite against this PR by commenting:

/benchmark

Optional regression threshold override (Δ% on Time or Alloc/op; defaults to 10%):

/benchmark threshold=15

Only one benchmark run per PR is active at a time — issuing a new /benchmark comment cancels the previous run. After the run finishes a separate comment will be posted comparing it against the latest scheduled run on main; the PR check fails if any benchmark regresses by more than the threshold.