Set up Cloud Agent dev environment + document caveats in AGENTS.md

[arloor]

Summary

Sets up the Cloud Agent development environment for rust_http_proxy and records durable, non-obvious caveats for future agents in AGENTS.md.

No application/source code was modified — only AGENTS.md gains a ## Cursor Cloud specific instructions section.

Environment setup performed

• Installed rustup stable (1.97.1) and set it as the default, because the crate is edition = "2024" (MSRV 1.85) but the base image ships rustc 1.83.
• Installed eBPF build deps (autoconf autopoint flex bison gawk clang pkg-config make libelf-dev zlib1g-dev) so the optional bpf* features build (--all-features).
• Update script proposed: cargo fetch --locked (minimal, idempotent dependency refresh).

Verification

Check	Command	Result
Build	cargo build	Pass
Format	cargo fmt --all -- --check	Pass
Lint	cargo clippy -- -D warnings	Pass (lib + bins)
Tests	cargo test	100 passed / 6 pre-existing H2 failures
Run	./target/debug/rust_http_proxy -p 7788 --web-content-path ... --users ...	Static serving, forward proxy (407 + authed fetch), and /metrics all work

Known pre-existing test failures

6 HTTP/2-upstream e2e tests fail with hyper::Error(Io, BrokenPipe). Trace logs confirm the proxy actually returns 200 OK; the failure is a race during HTTP/2 connection teardown. CI never runs cargo test, so this is ungated and unrelated to environment setup. Documented in AGENTS.md.