docs: add full command reference; fix module path and KernelURL config

- Add docs/commands.md with per-command purpose, step-by-step shell/SDK
  call sequences, config tables, outputs, and error conditions
- Rename module from github.com/you/fc-orchestrator to github.com/kacerr/fc-orchestrator
- Add KernelURL field to Config so the download URL is configurable via
  FC_KERNEL_URL instead of being hardcoded in Init()
- Expose FC_KERNEL_URL in the usage string
- Add verbose logging of dd/mkfs.ext4/mount/tar calls in buildRootfs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

orchestrator/config.go

@@ -10,6 +10,7 @@ type Config struct {
 	FCBin      string // path to firecracker binary
 	BaseDir    string // working directory for all state
 	Kernel     string // path to vmlinux
+	KernelURL  string // URL to download vmlinux if Kernel file is missing
 	Rootfs     string // path to base rootfs.ext4
 	VCPUs      int64
 	MemMiB     int64
@@ -33,6 +34,8 @@ func DefaultConfig() Config {
 		BootArgs:    "console=ttyS0 reboot=k panic=1 pci=off i8042.noaux quiet loglevel=0",
 	}
 	c.Kernel = envOr("FC_KERNEL", c.BaseDir+"/vmlinux")
+	c.KernelURL = envOr("FC_KERNEL_URL",
+		"https://s3.amazonaws.com/spec.ccfc.min/firecracker-ci/20260408-ce2a467895c1-0/x86_64/vmlinux-6.1.166")
 	c.Rootfs = envOr("FC_ROOTFS", c.BaseDir+"/rootfs.ext4")
 	return c
 }