docs: add Cursor Cloud environment setup notes to AGENTS.md

[nahime0]

What

Sets up and verifies the Cloud Agent development environment for elephc (PHP-to-native compiler in Rust) and records durable, non-obvious cloud caveats in AGENTS.md.

Environment setup

• Installed Rust 1.95.0 (CI-pinned in .github/docker/ci.Dockerfile) and set it as the rustup default (base image had 1.83.0).
• Installed missing system libraries: libbz2-dev, libssl-dev (bridge-crate deps). as/ld/gcc already present.
• Installed cargo-nextest 0.9.140 (used by CI and referenced in AGENTS.md).
• Update script: cargo build — rebuilds the compiler and materializes the bridge libelephc_*.a staticlibs.

Verification (end-to-end)

• Clean cargo build (zero warnings).
• Compiled and ran examples/fizzbuzz/main.php → native binary produces correct output.
• Focused test passed: cargo test --test codegen_tests fizzbuzz.

elephc_env_verification.log

Changes

• AGENTS.md: added a ## Cursor Cloud specific instructions section (toolchain pinning, host as/gcc benign linker warnings, php not installed, Docker cross-test scripts unavailable, quick sanity check).

No source code changed.

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