Add AGENTS.md file

AGENTS.md

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+# AGENTS.md
+
+## What This Project Is
+
+`bwai` is a CLI tool that runs AI coding agents (Claude, Gemini, Goose) inside a [bubblewrap](https://github.com/containers/bubblewrap) sandbox. The sandbox enforces a read-only host filesystem, whitelisted dotfiles and environment variables, and restricts writes to the current working directory only — preventing agents from accessing sensitive data like SSH keys, AWS credentials, or GPG keys.
+
+## Project Layout
+
+```
+cmd/bwai/
+  main.go       # Entry point, flag parsing, bwrap command construction
+  config.go     # Config struct, defaults, JSON loading (~/.bwai.json)
+  mounts.go     # Filesystem mount logic (home, DNS, GPU, shm)
+  bwrap.go      # Low-level bwrap argument helpers (roBind, rwBind, devBind, tmpfs)
+  update.go     # Self-update from GitHub releases with SHA-256 verification
+  version.go    # Version constant (set at build time via ldflags)
+  *_test.go     # Unit tests for each module
+scripts/
+  check.sh      # Runs fmt, lint, and tests (used in CI and pre-commit)
+Makefile        # build, install, test, fmt, lint, clean targets
+```
+
+## Language and Dependencies
+
+- Pure Go 1.21, zero external dependencies (stdlib only)
+- Requires `bwrap` installed on the host system (Linux only)
+
+## Build and Test
+
+```sh
+make build    # Compiles to bin/bwai
+make test     # Runs all unit tests
+make lint     # Runs golangci-lint
+make fmt      # Formats code
+```
+
+CI runs `scripts/check.sh` which chains fmt-check, lint, and test.
+
+## Configuration
+
+The config file lives at `~/.bwai.json`. Key fields:
+
+| Field | Purpose |
+|---|---|
+| `bwrap_path` | Path to the `bwrap` binary |
+| `bwrap_extra_args` | Extra flags passed directly to bwrap |
+| `command` | Default command to run inside the sandbox |
+| `home_allow` | Dotfiles/paths from `$HOME` to expose (read-only) |
+| `home_block` | Dotfiles/paths to explicitly block |
+| `env_allow` | Environment variables to pass through |
+
+`home_allow` takes precedence over `home_block`. Patterns support glob suffixes (e.g., `.bash_history*`) and nested paths (e.g., `.config/goose`).
+
+## Key Design Decisions
+
+- **Read-only by default**: The entire host OS tree is bind-mounted read-only. Only the current working directory is writable.
+- **No parent-process exit issue**: Non-bash commands are wrapped in `bash -i -c` to avoid agent processes becoming orphaned when the shell exits.
+- **GPU support**: `gpuMounts()` in `mounts.go` auto-detects and mounts NVIDIA and DRI devices.
+- **Safe self-update**: Downloads binary, verifies SHA-256 digest, replaces atomically with rollback on failure.
+- **Zero dependencies**: Keeps the supply chain minimal; everything uses Go stdlib.
+
+## CLI Flags
+
+```
+bwai                        # Open sandboxed bash shell
+bwai -c claude              # Launch claude directly inside sandbox
+bwai --ro-dir /some/path    # Expose extra read-only directory
+bwai -- --some-agent-flag   # Pass args to the configured command
+bwai update                 # Self-update to latest GitHub release
+bwai --dump-config          # Print default config as JSON
+```
+
+## Adding New Features
+
+- **New mount type**: Add a helper function in `mounts.go` following the existing pattern (`[]string` return, appended in `main.go`).
+- **New CLI flag**: Add to `main.go` using the `flag` package; wire into the bwrap args slice.
+- **New config field**: Add to the `Config` struct in `config.go` and update `defaultConfig()`.
+- **New agent support**: Add its dotfiles to `home_allow` defaults and its API key to `env_allow` defaults in `config.go`.
+
+## Testing
+
+Tests live alongside source files (`*_test.go`). They use only `testing` from stdlib. Run with `make test`. There is no integration test harness — unit tests cover mount logic, bwrap arg construction, and the update mechanism.