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/daemon/doctor.go

@@ -2,6 +2,7 @@ package daemon
 
 import (
 	"context"
+	"fmt"
 	"strings"
 
 	"banger/internal/config"
@@ -40,11 +41,34 @@ func (d *Daemon) doctorReport(ctx context.Context) system.Report {
 	report.AddPreflight("host runtime", d.runtimeChecks(), runtimeStatus(d.config))
 	report.AddPreflight("core vm lifecycle", d.coreVMLifecycleChecks(), "required host tools available")
 	report.AddPreflight("vsock guest agent", d.vsockChecks(), "vsock guest agent prerequisites available")
+	d.addVMDefaultsCheck(&report)
 	d.addCapabilityDoctorChecks(ctx, &report)
 
 	return report
 }
 
+// addVMDefaultsCheck surfaces the effective VM sizing that `vm run` /
+// `vm create` will apply when the user omits the flags. Shown as a
+// PASS check so it always renders, with per-field provenance
+// (config|auto|builtin) so users can tell what's driving each number.
+func (d *Daemon) addVMDefaultsCheck(report *system.Report) {
+	host, err := system.ReadHostResources()
+	var cpus int
+	var memBytes int64
+	if err == nil {
+		cpus = host.CPUCount
+		memBytes = host.TotalMemoryBytes
+	}
+	defaults := model.ResolveVMDefaults(d.config.VMDefaults, cpus, memBytes)
+	details := []string{
+		fmt.Sprintf("vcpu:   %d (%s)", defaults.VCPUCount, defaults.VCPUSource),
+		fmt.Sprintf("memory: %d MiB (%s)", defaults.MemoryMiB, defaults.MemorySource),
+		fmt.Sprintf("disk:   %s (%s)", model.FormatSizeBytes(defaults.WorkDiskSizeBytes), defaults.WorkDiskSource),
+		"override any of these in ~/.config/banger/config.toml under [vm_defaults]",
+	}
+	report.AddPass("vm defaults", details...)
+}
+
 func (d *Daemon) runtimeChecks() *system.Preflight {
 	checks := system.NewPreflight()
 	checks.RequireExecutable(d.config.FirecrackerBin, "firecracker binary", `install firecracker or set "firecracker_bin"`)