docs: add agent instructions

AGENTS.md

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+# AGENTS.md
+
+Instructions for coding agents working in this repository.
+
+## Project Shape
+
+`gosleep-timer` is a Rust-first, single-binary Linux terminal app.
+
+- Main code: `src/main.rs`
+- Package metadata: `Cargo.toml`
+- Locked dependency graph: `Cargo.lock`
+- Version files: `Cargo.toml`, `Cargo.lock`, `VERSION`, `CHANGELOG.md`
+- CI: `.gitea/workflows/ci.yaml`
+- Release workflow: `.gitea/workflows/release.yaml`
+- Release docs: `RELEASING.md`
+
+Do not reintroduce Go, Python, Node, Docker packaging, or multi-language runtime dependencies unless the user explicitly asks for that migration.
+
+## Required Checks
+
+Run these before committing code changes:
+
+```bash
+cargo fmt --check
+cargo test --locked
+cargo clippy --locked -- -D warnings
+cargo build --release --locked
+```
+
+Run YAML lint when editing workflow or YAML files:
+
+```bash
+yamllint -c .yamllint .
+```
+
+If `yamllint` is not installed, the CI-compatible fallback is:
+
+```bash
+docker run --rm -v "$PWD:/work" -w /work cytopia/yamllint:latest -c .yamllint .
+```
+
+## Local Smoke Tests
+
+Use a temporary config path so local tests do not mutate the user's real config:
+
+```bash
+tmpdir="$(mktemp -d)"
+cargo run -- --config "$tmpdir/config.yaml" init
+cargo run -- --config "$tmpdir/config.yaml" preview
+```
+
+For `run` smoke tests, disable actions first so the command does not call `playerctl`, `systemctl`, `pkill`, workspace tools, or custom shell commands.
+
+## Versioning
+
+The project uses semantic versioning. Tags must be annotated and formatted as `vMAJOR.MINOR.PATCH`.
+
+When bumping a version, update all of:
+
+- `Cargo.toml`
+- `Cargo.lock`
+- `VERSION`
+- `CHANGELOG.md`
+
+Verify version consistency with:
+
+```bash
+test "$(cat VERSION)" = "$(cargo metadata --no-deps --format-version 1 | jq -r '.packages[0].version')"
+```
+
+## Releases
+
+Release workflow is tag-based:
+
+```bash
+git tag -a v1.1.1 -m "gosleep-timer v1.1.1"
+git push origin main
+git push origin v1.1.1
+```
+
+The Gitea release workflow:
+
+- builds `target/release/gosleep-timer`
+- creates `gosleep-timer-<version>-linux-amd64.tar.gz`
+- writes a `.sha256`
+- creates or reuses the matching Gitea release
+- replaces release assets with matching names
+
+The workflow uses the built-in `GITEA_TOKEN` and declares `permissions: contents: write`.
+
+## Git Remote
+
+Expected upstream:
+
+```text
+origin ssh://git@gitssh.forust.xyz:2221/forust/gosleep.git
+```
+
+HTTPS remote URL:
+
+```text
+https://gitea.forust.xyz/forust/gosleep.git
+```
+
+Before pushing, confirm:
+
+```bash
+git status --short --branch
+git branch -vv
+git remote -v
+```
+
+## Repository Hygiene
+
+- Keep `target/`, `dist/`, `.env*`, logs, temp files, and local artifacts out of git.
+- Keep `Cargo.lock` committed; this is an application, not a library.
+- Prefer small, direct Rust changes in `src/main.rs` until the code genuinely needs splitting.
+- Do not add broad abstractions just to prepare for hypothetical modules.
+- Do not add shelling out during tests unless the test fully controls the command.
+- Keep docs synchronized with actual behavior and command names.
+- Keep release assets and generated archives out of the repo.
+
+## TUI Notes
+
+The TUI uses Ratatui and Crossterm. Be careful with terminal-size assumptions.
+
+For layout changes, check both:
+
+- narrow terminal behavior, where settings and preview stack vertically
+- wide terminal behavior, where settings and preview are side by side
+
+Avoid truncating command previews without a visible indication. Preview wrapping is expected behavior.
+
+## Safety
+
+The app can run destructive or disruptive post-countdown commands:
+
+- `pkill`
+- `systemctl poweroff`
+- `systemctl reboot`
+- arbitrary custom shell commands
+
+Never run `gosleep-timer run` against the real user config during verification unless the user explicitly asks for it. Use an isolated config and disable actions for smoke tests.