AGENTS.md: make the check-the-official-clients practice a hard rule

The practice existed - go-parity.md is prefaced with it, sdk-comparison.md
exists to record deviations from it, retry.rs follows the Python client's
retry list, and #38/#39/#40 all ran on it - but the Hard rules section only
covered protocol facts (docs + a real cluster). Behavioural decisions the
protocol does not dictate (retries, routing, proxy selection, banning,
refresh) had no written rule, so following the official C++/Go clients was
convention rather than specification.

Rule 3 now carries both halves: protocol facts against docs and cluster,
behaviour against the official clients' source, disagreements between the
two clients resolved out loud, and deviations recorded in sdk-comparison.md
- the record that turned the heavy-proxy lifetime pin from a silent bug into
issue #40 with a known fix.

Author: sshaplygin

AGENTS.md

@@ -56,6 +56,18 @@ repository builds the minimal stack — a YSON codec and a job runtime.
 3. Protocol facts are verified against the official YTsaurus documentation and
    against a real cluster. If code and docs disagree, **re-read the docs first**,
    then change the code. Cite the doc at the point of use.
+   **Client behaviour the protocol does not dictate — retries, routing, proxy
+   selection, banning, refresh — is checked against the official clients'
+   source before it is designed here**: C++ (`yt/cpp/mapreduce`) and Go
+   (`yt/go`), and the Python wrapper where it is the reference (the retry
+   list). Where the two disagree, say which was followed and why; where this
+   client deviates from both, the deviation is a deliberate decision recorded
+   in [docs/sdk-comparison.md](docs/sdk-comparison.md) — that record is what
+   turned the heavy-proxy lifetime pin from a silent bug into #40, a filed
+   divergence with a known fix. [docs/go-parity.md](docs/go-parity.md) is the
+   same rule for API surface: read it before adding client API, because a
+   feature list written by the people who built the thing is worth more than
+   one written by the people reimplementing it.
 4. Every change ends with green CI: `cargo fmt --check`, `cargo clippy
    --all-targets -D warnings`, `cargo test`, `cargo test --doc`.
 5. **No scope creep.** RPC proxy, protobuf row format, dynamic tables, non-Linux