Refactor VM lifecycle around capabilities

Make host-integrated VM features fit a standard Go extension path instead of adding more one-off branches through vm.go. This is the enabling refactor for future work like shared mounts, not the /work feature itself.

Add a daemon capability pipeline plus a structured guest-config builder, then move the existing /root work-disk mount, built-in DNS, and NAT wiring onto those hooks. Generalize Firecracker drive config at the same time so later storage features can extend machine setup without another hardcoded path.

Add banger doctor on top of the shared readiness checks, update the docs to describe the new architecture, and cover the new seams with guest-config, capability, report, CLI, and full go test verification. Also verify make build and a real ./banger doctor run on the host.

internal/guestconfig/guestconfig.go

@@ -0,0 +1,159 @@
+package guestconfig
+
+import (
+	"sort"
+	"strconv"
+	"strings"
+)
+
+type MountSpec struct {
+	Source  string
+	Target  string
+	FSType  string
+	Options []string
+	Dump    int
+	Pass    int
+}
+
+func (m MountSpec) String() string {
+	options := strings.Join(compactStrings(m.Options), ",")
+	if options == "" {
+		options = "defaults"
+	}
+	return strings.Join([]string{
+		m.Source,
+		m.Target,
+		m.FSType,
+		options,
+		strconv.Itoa(m.Dump),
+		strconv.Itoa(m.Pass),
+	}, " ")
+}
+
+type Builder struct {
+	files       map[string][]byte
+	dropTargets map[string]struct{}
+	mounts      map[string]MountSpec
+	order       []string
+}
+
+func NewBuilder() *Builder {
+	return &Builder{
+		files:       make(map[string][]byte),
+		dropTargets: make(map[string]struct{}),
+		mounts:      make(map[string]MountSpec),
+	}
+}
+
+func (b *Builder) WriteFile(path string, data []byte) {
+	if b.files == nil {
+		b.files = make(map[string][]byte)
+	}
+	b.files[path] = append([]byte(nil), data...)
+}
+
+func (b *Builder) DropMountTarget(target string) {
+	target = strings.TrimSpace(target)
+	if target == "" {
+		return
+	}
+	if b.dropTargets == nil {
+		b.dropTargets = make(map[string]struct{})
+	}
+	b.dropTargets[target] = struct{}{}
+}
+
+func (b *Builder) AddMount(spec MountSpec) {
+	spec.Source = strings.TrimSpace(spec.Source)
+	spec.Target = strings.TrimSpace(spec.Target)
+	spec.FSType = strings.TrimSpace(spec.FSType)
+	spec.Options = compactStrings(spec.Options)
+	if spec.Source == "" || spec.Target == "" || spec.FSType == "" {
+		return
+	}
+	if b.mounts == nil {
+		b.mounts = make(map[string]MountSpec)
+	}
+	if _, exists := b.mounts[spec.Target]; !exists {
+		b.order = append(b.order, spec.Target)
+	}
+	b.mounts[spec.Target] = spec
+}
+
+func (b *Builder) Files() map[string][]byte {
+	if len(b.files) == 0 {
+		return nil
+	}
+	keys := b.FilePaths()
+	out := make(map[string][]byte, len(keys))
+	for _, path := range keys {
+		out[path] = append([]byte(nil), b.files[path]...)
+	}
+	return out
+}
+
+func (b *Builder) FilePaths() []string {
+	keys := make([]string, 0, len(b.files))
+	for path := range b.files {
+		keys = append(keys, path)
+	}
+	sort.Strings(keys)
+	return keys
+}
+
+func (b *Builder) RenderFSTab(existing string) string {
+	lines := strings.Split(existing, "\n")
+	out := make([]string, 0, len(lines)+len(b.mounts))
+	managedTargets := make(map[string]struct{}, len(b.mounts))
+	for target := range b.mounts {
+		managedTargets[target] = struct{}{}
+	}
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		if trimmed == "" {
+			continue
+		}
+		target := mountTarget(trimmed)
+		if target != "" {
+			if _, drop := b.dropTargets[target]; drop {
+				continue
+			}
+			if _, managed := managedTargets[target]; managed {
+				continue
+			}
+		}
+		out = append(out, line)
+	}
+	for _, target := range b.order {
+		out = append(out, b.mounts[target].String())
+	}
+	return strings.Join(out, "\n") + "\n"
+}
+
+func mountTarget(line string) string {
+	fields := strings.Fields(line)
+	if len(fields) < 2 {
+		return ""
+	}
+	return fields[1]
+}
+
+func compactStrings(values []string) []string {
+	if len(values) == 0 {
+		return nil
+	}
+	out := make([]string, 0, len(values))
+	seen := make(map[string]struct{}, len(values))
+	for _, value := range values {
+		value = strings.TrimSpace(value)
+		if value == "" {
+			continue
+		}
+		if _, ok := seen[value]; ok {
+			continue
+		}
+		seen[value] = struct{}{}
+		out = append(out, value)
+	}
+	return out
+}