docs: add AGENTS.md repository guidelines

Author: dedene

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure
+
+- `cmd/lametric/`: CLI entrypoint
+- `internal/`: implementation
+  - `cmd/`: command routing (kong CLI framework)
+  - `api/`: LaMetric device API client
+  - `auth/`: keyring-based credential storage
+  - `config/`: YAML configuration
+  - `discovery/`: mDNS/SSDP device discovery
+  - `output/`: terminal rendering (lipgloss)
+  - `errfmt/`: error formatting
+- `bin/`: build outputs
+
+## Build, Test, and Development Commands
+
+- `make build`: compile to `bin/lametric`
+- `make lametric -- <args>`: build + run (e.g., `make lametric -- notify "Hello"`)
+- `make fmt` / `make lint` / `make test` / `make ci`: format, lint, test, full local gate
+- `make tools`: install pinned dev tools into `.tools/`
+- `make clean`: remove bin/ and .tools/
+
+## Coding Style & Naming Conventions
+
+- Formatting: `make fmt` (goimports local prefix `github.com/dedene/lametric-cli` + gofumpt)
+- Output: keep stdout parseable (`--json`); send human hints/progress to stderr
+- Linting: golangci-lint v2.8.0 with project config
+
+## Testing Guidelines
+
+- Unit tests: stdlib `testing` (files: `*_test.go` next to code)
+- 11 test files; comprehensive coverage
+- CI gate: fmt-check, lint, test
+
+## Config & Secrets
+
+- **Keyring**: 99designs/keyring for API key storage (macOS Keychain, Linux SecretService, file backend)
+- **Env var fallback**: `LAMETRIC_API_KEY`
+- **Keyring backend**: `LAMETRIC_KEYRING_BACKEND` (auto/keychain/file)
+- **Keyring password**: `LAMETRIC_KEYRING_PASSWORD` (for file backend)
+- **Device discovery**: automatic via mDNS/SSDP on local network
+
+## Key Features
+
+- Send notifications to LaMetric TIME displays
+- Device discovery on local network
+- Multiple device support
+
+## Commit & Pull Request Guidelines
+
+- Conventional Commits: `feat|fix|refactor|build|ci|chore|docs|style|perf|test`
+- Group related changes; avoid bundling unrelated refactors
+- PR review: use `gh pr view` / `gh pr diff`; don't switch branches
+
+## Security Tips
+
+- Never commit API keys
+- Prefer OS keychain; file backend only for headless environments
+- Device API keys should be unique per device

CLAUDE.md

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+@AGENTS.md