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.

README.md

@@ -134,6 +134,28 @@ Most commonly set:
 
 Full key list in `internal/config/config.go`.
 
+### `vm_defaults` — sizing for new VMs
+
+Every `vm run` / `vm create` prints a `spec:` line up front showing
+the vCPU, RAM, and disk the VM will get. When the flags aren't set,
+those values come from:
+
+1. `[vm_defaults]` in config (if present, wins).
+2. Host-derived heuristics (roughly: `cpus/4` capped at 4, `ram/8`
+   capped at 8 GiB, 8 GiB disk).
+3. Built-in constants (floor).
+
+`banger doctor` prints the effective defaults with provenance.
+
+```toml
+[vm_defaults]
+vcpu = 4
+memory_mib = 4096
+disk_size = "16G"
+```
+
+All keys optional — omit whichever you want banger to decide.
+
 ### `file_sync` — host → guest file copies
 
 ```toml