vm defaults: host-aware sizing + spec line on spawn + doctor check

Replaces the static model.Default* constants that drove --vcpu / --memory
/ --disk-size with a three-layer resolver:

  1. [vm_defaults] in ~/.config/banger/config.toml (if set)
  2. host-derived heuristics (cpus/4 capped at 4; ram/8 capped at 8 GiB)
  3. baked-in constants (floor)

Visibility:

- Every `vm run` / `vm create` prints a `spec:` line before progress
  begins: `spec: 4 vcpu · 8192 MiB · 8G disk`. Matches the VM that
  actually gets created because the CLI is now the single source of
  truth — it resolves, populates the flag defaults, and forwards the
  explicit values to the daemon.
- `banger doctor` adds a "vm defaults" check showing per-field
  provenance (config|auto|builtin) and the config file path for
  overrides.
- `--help` shows the resolved defaults (e.g. `--vcpu int (default 4)`
  on an 8-core host).

No `banger config init` command, no first-run side effects, no writes
to the user's filesystem behind their back. Users who want explicit
control set the keys; everyone else gets sensible numbers that track
their hardware.

internal/config/config_test.go

@@ -205,3 +205,65 @@ func TestLoadRejectsInvalidFileSyncEntries(t *testing.T) {
 		})
 	}
 }
+
+func TestLoadAcceptsVMDefaults(t *testing.T) {
+	configDir := t.TempDir()
+	data := []byte(`
+[vm_defaults]
+vcpu = 4
+memory_mib = 4096
+disk_size = "16G"
+system_overlay_size = "12G"
+`)
+	if err := os.WriteFile(filepath.Join(configDir, "config.toml"), data, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(paths.Layout{ConfigDir: configDir})
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.VMDefaults.VCPUCount != 4 {
+		t.Errorf("VCPUCount = %d, want 4", cfg.VMDefaults.VCPUCount)
+	}
+	if cfg.VMDefaults.MemoryMiB != 4096 {
+		t.Errorf("MemoryMiB = %d, want 4096", cfg.VMDefaults.MemoryMiB)
+	}
+	if cfg.VMDefaults.WorkDiskSizeBytes != 16*1024*1024*1024 {
+		t.Errorf("WorkDiskSizeBytes = %d, want 16 GiB", cfg.VMDefaults.WorkDiskSizeBytes)
+	}
+	if cfg.VMDefaults.SystemOverlaySizeByte != 12*1024*1024*1024 {
+		t.Errorf("SystemOverlaySizeByte = %d, want 12 GiB", cfg.VMDefaults.SystemOverlaySizeByte)
+	}
+}
+
+func TestLoadEmptyVMDefaultsLeavesZeros(t *testing.T) {
+	// No [vm_defaults] block → cfg.VMDefaults is the zero value,
+	// which the resolver will map to auto or builtin.
+	cfg, err := Load(paths.Layout{ConfigDir: t.TempDir()})
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.VMDefaults.VCPUCount != 0 || cfg.VMDefaults.MemoryMiB != 0 {
+		t.Errorf("VMDefaults = %+v, want zeroed", cfg.VMDefaults)
+	}
+}
+
+func TestLoadRejectsNegativeVMDefaults(t *testing.T) {
+	cases := map[string]string{
+		"vcpu":      `[vm_defaults]` + "\n" + `vcpu = -1`,
+		"memory":    `[vm_defaults]` + "\n" + `memory_mib = -1`,
+		"disk_size": `[vm_defaults]` + "\n" + `disk_size = "banana"`,
+		"overlay":   `[vm_defaults]` + "\n" + `system_overlay_size = "banana"`,
+	}
+	for name, body := range cases {
+		t.Run(name, func(t *testing.T) {
+			configDir := t.TempDir()
+			if err := os.WriteFile(filepath.Join(configDir, "config.toml"), []byte(body+"\n"), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			if _, err := Load(paths.Layout{ConfigDir: configDir}); err == nil {
+				t.Fatal("expected error")
+			}
+		})
+	}
+}