Add `gh devlake stop` command

[ewega]

Summary

Add a gh devlake stop command that gracefully stops DevLake services without destroying containers, volumes, or state. This is the counterpart to gh devlake start (#127) and distinct from gh devlake cleanup which tears everything down permanently.

Mental model for users:

• start — bring services up (safe, idempotent)
• stop — pause services (safe, non-destructive, data preserved)
• cleanup — permanent teardown (removes containers, volumes, files)

Proposed Solution

Local deployments (Docker Compose)

1. Auto-detect deployment from state file (.devlake-local.json) or docker-compose.yml in cwd
2. Run docker compose stop (graceful stop — preserves containers and volumes for quick restart)
3. Print confirmation

Azure deployments (Container Instances)

1. Read container names + resource group from .devlake-azure.json
2. Check Azure CLI login
3. Run az container stop for each container
4. Print confirmation

Key design decisions

• Uses docker compose stop (not docker compose down) — preserves containers and volumes for fast restart via start
• State file is NOT modified — Docker/Azure is the source of truth for runtime state
• No confirmation prompt needed (unlike cleanup) since the operation is non-destructive

Prerequisites: ComposeStop in docker package

Add ComposeStop(dir string) error to internal/docker/build.go:

func ComposeStop(dir string) error {
    cmd := execCommand("docker", "compose", "stop")
    cmd.Dir = dir
    if out, err := cmd.CombinedOutput(); err != nil {
        return fmt.Errorf("docker compose stop failed: %s\n%s", err, string(out))
    }
    return nil
}

Flags

Flag	Default	Description
--service <name>	(all)	Stop only a specific service (e.g., --service grafana)

Scope of Changes

File	Change
internal/docker/build.go	Add ComposeStop() function
internal/docker/docker_test.go	Add test for ComposeStop (mock pattern exists)
cmd/stop.go	New command implementation
cmd/deploy.go	Register stop in operate command group
docs/stop.md	New documentation
docs/day-2.md	Add stop to operations list
README.md	Add to command reference table

Acceptance Criteria

• gh devlake stop gracefully stops all local services
• gh devlake stop --service grafana stops only Grafana
• gh devlake stop works for Azure deployments (reads state file)
• Containers and volumes preserved after stop (verifiable with docker ps -a)
• gh devlake start brings everything back up cleanly after stop
• Error message guides user when no state file or docker-compose.yml found

Dependencies

• Blocked by: gh devlake start (Add gh devlake start command #127) — should be implemented together for lifecycle completeness

References

• internal/docker/build.go — ComposeDown() and ComposeUp() patterns
• cmd/cleanup.go — runLocalCleanup() uses ComposeDown (destructive counterpart)