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/cli/vm_spec_test.go

@@ -0,0 +1,53 @@
+package cli
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"banger/internal/api"
+)
+
+func TestPrintVMSpecLineWithAllFields(t *testing.T) {
+	vcpu, mem := 2, 2048
+	params := api.VMCreateParams{
+		VCPUCount:    &vcpu,
+		MemoryMiB:    &mem,
+		WorkDiskSize: "8G",
+	}
+	var buf bytes.Buffer
+	printVMSpecLine(&buf, params)
+	got := buf.String()
+	for _, want := range []string{"spec:", "2 vcpu", "2048 MiB", "8G"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("output missing %q:\n%s", want, got)
+		}
+	}
+	if !strings.HasSuffix(got, "\n") {
+		t.Error("spec line should terminate with newline")
+	}
+}
+
+func TestPrintVMSpecLineFallsBackToBuiltinsOnNilFields(t *testing.T) {
+	// Empty params — the printer reaches for DefaultVCPUCount /
+	// DefaultMemoryMiB / DefaultWorkDiskSize so output is still sane.
+	var buf bytes.Buffer
+	printVMSpecLine(&buf, api.VMCreateParams{})
+	got := buf.String()
+	// Not asserting exact values — just that it produced a plausible
+	// line with the three labels.
+	for _, want := range []string{"spec:", "vcpu", "MiB", "disk"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("output missing %q:\n%s", want, got)
+		}
+	}
+}
+
+func TestPrintVMSpecLineIgnoresUnparseableDiskSize(t *testing.T) {
+	// Falls back to builtin default; must not panic or print garbage.
+	var buf bytes.Buffer
+	printVMSpecLine(&buf, api.VMCreateParams{WorkDiskSize: "not-a-size"})
+	if !strings.Contains(buf.String(), "spec:") {
+		t.Errorf("expected spec line even with bad input, got %q", buf.String())
+	}
+}