docs: add concise AGENTS development guide

AGENTS.md

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+# AGENTS.md
+Concise maintainer/developer guide for building, testing, and opening high-quality PRs in this repo.
+
+## Goal (pick one per PR)
+- Make CLI better: improve UX, error messages, help text, flags, and output clarity.
+- Improve reliability: fix bugs, edge cases, and regressions with tests.
+- Improve developer velocity: simplify code paths, reduce complexity, keep behavior explicit.
+- Improve quality gates: strengthen tests/lint/checks without adding heavy process.
+
+## Fast Dev Loop
+1. `make build` (runs `python3 scripts/fetch_meta.py` first)
+2. `make unit-test` (required before PR)
+3. Run changed command(s) manually via `./lark-cli ...`
+
+## Pre-PR Checks (match CI gates)
+1. `make unit-test`
+2. `go mod tidy` (must not change `go.mod`/`go.sum`)
+3. `go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.1.6 run --new-from-rev=origin/main`
+4. If dependencies changed: `go run github.com/google/go-licenses/v2@v2.0.1 check ./... --disallowed_types=forbidden,restricted,reciprocal,unknown`
+5. Optional full local suite: `make test` (vet + unit + integration)
+
+## Test/Check Commands
+- Unit: `make unit-test`
+- Integration: `make integration-test`
+- Full: `make test`
+- Vet only: `make vet`
+- Coverage (local): `go test -race -coverprofile=coverage.txt -covermode=atomic ./...`
+
+## Commit/PR Rules
+- Use Conventional Commits in English: `feat: ...`, `fix: ...`, `docs: ...`, `ci: ...`, `test: ...`, `chore: ...`, `refactor: ...`
+- Keep PR title in the same Conventional Commit format (squash merge keeps it).
+- Before opening a real PR, draft/fill description from `.github/pull_request_template.md` and ensure Summary/Changes/Test Plan are complete.
+- Never commit secrets/tokens/internal sensitive data.