Subagent skills (`/explore`, `/research`, `/review`, `/security-review`
out of the box, plus any user-authored `runAs: subagent` skill) had only
one knob for picking the spawn model: a `model:` line in the .md
frontmatter. That meant users couldn't tune the four built-ins without
shadowing them by copying the bodies into a custom file, and the desktop
/ web settings had no surface for it at all.

Adds a per-skill `flash | pro` selector:
- `config.json` gets `subagentModels?: Record<skillName, "flash" | "pro">`
  with `load`/`save` helpers that filter to valid values
- `SkillStore` accepts `subagentModels` in its constructor and applies
  the override only to `runAs: subagent` entries (inline skills with
  the same name are untouched, frontmatter `model:` is overridden
  when an entry exists)
- `registerSkillTools` plumbs it through; `buildCodeToolset` reads
  `loadSubagentModels()` so the runtime path picks up the live value
- desktop `settings_save` accepts the field, persists via
  `saveSubagentModels`, re-emits the skill list so the UI's
  inline-rendered model badge stays in sync
- dashboard `POST /api/settings` validates the same shape, GET surfaces
  the current map, `appliesAt.subagentModels` = "next-skill-run"
- both desktop and dashboard Settings → Skills page render a
  `<select flash | pro>` per subagent skill — inline skills get no
  selector

`flash`/`pro` map to `deepseek-v4-flash`/`deepseek-v4-pro` inside
`SkillStore` so the config stays human-readable across model id swaps.

Tests:
- config: round-trip, filter unknown values, clear-all removes field
- SkillStore: override applies to builtins, doesn't touch inline `test`,
  override beats frontmatter `model:` when both set, frontmatter still
  wins absent override

CodeQL js/remote-property-injection — `__proto__` / `constructor` /
`prototype` in a POST /api/settings body would land as own properties
on the sanitized map and persist into config.json.