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/model/types.go

@@ -66,6 +66,7 @@ type DaemonConfig struct {
 	DefaultDNS          string
 	DefaultImageName    string
 	FileSync            []FileSyncEntry
+	VMDefaults          VMDefaultsOverride
 }
 
 // FileSyncEntry is a user-declared host→guest file or directory copy