Add AGENTS.md with project-specific guidance for opencode sessions

AGENTS.md

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+# AGENTS.md
+
+## Project overview
+
+Docker image packaging [opencode](https://opencode.ai) into a production-ready dev container. Two image variants (base and omos) are published to Docker Hub via Gitea Actions CI. Not a library or application — this is infrastructure (Dockerfile, entrypoint scripts, docker-compose, documentation).
+
+## File roles
+
+- `Dockerfile` — single multi-stage build for both variants. OMOS variant is controlled by `INSTALL_OMOS=true` build arg. All GitHub-sourced binaries are pinned with version ARGs.
+- `entrypoint.sh` — runs as root: UID/GID adjustment, SSH permissions, volume ownership fixes. Then drops to developer via gosu.
+- `entrypoint-user.sh` — runs as developer: git config, opencode.json generation from env vars, OMOS setup.
+- `DOCKER_HUB.md` — pushed to Docker Hub description via CI API call. Must stay under 25KB. Short description field must be ≤100 bytes.
+- `README.md` — source repo documentation. Must stay in sync with DOCKER_HUB.md (both describe the same features but for different audiences).
+- `.gitea/workflows/docker-publish.yml` — CI pipeline: three parallel jobs (build-base, build-omos, update-description). Triggered by tag push only.
+
+## Versioning scheme
+
+Tags follow `v{opencode_version}{letter}` — e.g. `v1.4.3k`. The number matches the opencode npm version. The letter suffix increments for container-level changes (tooling, docs, CVE fixes) on the same opencode version. CI produces four Docker Hub tags per release: `vX.Y.Zn`, `latest`, `vX.Y.Zn-omos`, `latest-omos`.
+
+## Critical conventions
+
+- **entrypoint.sh volume ownership loop** — when adding a new named volume mount point, add it to the `for dir in ...` loop in `entrypoint.sh` so root-owned volumes get chowned on startup.
+- **Three docs to keep in sync** — Dockerfile changes that add tools or features must be reflected in `README.md`, `DOCKER_HUB.md`, and `.env.example`. The docker-compose examples in both docs must match the source `docker-compose.yml` pattern.
+- **GitHub-sourced binaries** — fzf, gosu, git-lfs, neovim, bat, eza, zoxide, uv, rustup are installed from upstream releases (not apt) with pinned versions. Use the same `ARCH` case-switch pattern for multi-arch support (amd64/arm64).
+- **Shell scripts use `set -euo pipefail`** — both entrypoints are strict. Errors in volume chown or SSH permission operations are intentionally suppressed with `|| true`.
+- **Docker Hub description update** — uses `/v2/auth/token` endpoint (not the deprecated `/v2/users/login`). Auth uses `identifier`/`secret` fields, returns `access_token`, sent as `Bearer`. Short description must be ≤100 bytes.
+
+## CI quirks
+
+- Both build jobs include an IPv4 preference step (`gai.conf` + `driver-opts: network=host` for buildx) to work around intermittent IPv6 failures on the Gitea runners.
+- `update-description` job runs only when both builds succeed (`needs: [build-base, build-omos]`).
+- Tags must be pushed to trigger CI. Pushing to `main` alone does not build images.
+
+## Testing changes
+
+No test suite. Verify by:
+1. Building locally: `docker compose build`
+2. Running: `docker compose run --rm devbox bash`
+3. Checking tool availability inside container: `nvim --version`, `bat --version`, `uv --version`, etc.
+4. For entrypoint changes: test with a non-1000 UID workspace to verify UID adjustment and volume ownership fixes.
+
+## Commit style
+
+Imperative mood, first line summarizes the change. Multi-line body explains "why" when non-obvious. Examples from history:
+- `Fix ownership of named volume mount points in entrypoint`
+- `Add uv package manager to base image for on-demand Python support`
+- `Upgrade base image from Debian bookworm to trixie (current stable)`