Add CLAUDE.md and AGENTS.md with build rules and pitfalls

AGENTS.md

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+# Agent guidelines for agent-sandbox
+
+## Build and test
+
+- `cargo fmt` and `cargo clippy` must pass before every commit.
+- `cargo test` runs all integration tests. Tests run serially (configured in `.cargo/config.toml`) because they spawn real bwrap sandboxes that share host paths like `/tmp`.
+- Never add Co-Authored-By lines to commits.
+
+## Things that will bite you
+
+### bwrap argument ordering matters
+
+Later bwrap arguments override earlier ones for the same path. This has caused multiple bugs:
+
+- Blacklist overlays (tmpfs, ro-bind /dev/null) must come **after** the base `--ro-bind / /` and `--bind /tmp /tmp`.
+- The `/run` tmpfs and its selective whitelisted binds must come **after** the overlay section, or the overlays clobber the whitelisted paths.
+- User `--rw`/`--ro` escape hatches must come **after** mode setup so they can override sandbox restrictions.
+
+Take extreme care when reordering any arguments in `sandbox.rs` or refactor things and test thoroughly.

CLAUDE.md

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+@AGENTS.md