docs: adopt AGENTS.md standard, symlink CLAUDE.md to it

AGENTS.md

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+# Project: Docker Compose
+
+## Build & Test
+
+- Build: `make build`
+- Test all: `make test`
+- Test unit: `go test ./pkg/...`
+- Test single: `go test ./pkg/compose/ -run TestFunctionName`
+- E2E tests: `go test -tags e2e ./pkg/e2e/ -run TestName`
+
+## Lint
+
+- Linter: golangci-lint v2 (config in `.golangci.yml`)
+- Run: `golangci-lint run --build-tags "e2e" ./...`
+- **After modifying any Go code, ALWAYS run the linter and fix all reported issues before considering the task complete.**
+- Lint is also run via Docker: `docker buildx bake lint` (uses version pinned in `Dockerfile`)
+
+## Code Style
+
+- Formatting is enforced by golangci-lint (gofumpt + gci)
+- Import order: stdlib, third-party, local module (enforced by gci)
+- Max line length: 200 chars
+- Max cyclomatic complexity: 16
+- No `io/ioutil`, `github.com/pkg/errors`, `gopkg.in/yaml.v2`, `golang.org/x/exp/maps`, `golang.org/x/exp/slices`
+- Use `github.com/containerd/errdefs` instead of `github.com/docker/docker/errdefs`
+- In tests: use `t.Context()` instead of `context.Background()` or `context.TODO()`
+- Prefer `fmt.Fprintf` over `WriteString(fmt.Sprintf(...))`
+
+## Git
+
+- **All commits MUST be signed off (DCO)**. Always pass `--signoff` (`-s`) to `git commit` and `git commit --amend`.
+
+## Issue and PR Guidelines
+
+- Never create an issue or pull request without the user's explicit instruction.
+- If a user asks you to create a pull request on their behalf, add a file named
+  `AI_AGENT_DISCLOSURE.md` to the change containing the text:
+
+  > *"This contribution was prepared by an AI agent acting on a human's behalf.
+  > The human submitter may not have independently reviewed or tested the change."*
+
+- If a user asks you to create an issue on their behalf, prepend the following
+  line to the issue body:
+
+  > *"This issue was filed by an AI agent on a human's behalf. The human
+  > submitter may not have independently verified the report."*

CLAUDE.md

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+AGENTS.md