[FEATURE] Let spawn_task specify the model for the spawned chip session (settings-file model is ignored)

Problem

Orchestration workflows in Claude Code Desktop route cheap, well-specified work to task chips created with the ccd_session MCP tool spawn_task. The natural setup is an expensive orchestrator model (Fable/Opus) spawning chips that should run on a cheaper model (Sonnet). Today there is no way to make that happen automatically:

• spawn_task accepts only title/tldr/prompt/cwd, no model parameter.
• Setting "model": "sonnet" in the repo root's .claude/settings.local.json (or .claude/settings.json) has no effect on chip sessions. Verified 2026-07-22: a chip spawned from a Fable orchestrator session started on claude-opus-4-8 and then ran turns on claude-fable-5 (per its transcript), never Sonnet. The spawner's model choice (app default or parent session's model) overrides settings files.
• Chip sessions run in app-managed worktrees under <repo>\.claude\worktrees\<name>, and their settings resolution there is undocumented; either the spawner passes an explicit model that outranks settings, or project settings are not resolved from the main checkout for app-managed worktrees.

The only workaround is the user manually switching the model in the desktop picker for every chip, which defeats one-click chip UX and, when forgotten, silently burns top-tier tokens on worker tasks (this is how we noticed: a mechanical diagnosis chip ran on Fable).

Request

Either (in order of preference):

1. A model parameter on spawn_task so the orchestrating session can pin the spawned session's model (validated against the same registry as the Agent tool's model override), or
2. A desktop app setting such as "default model for spawned/background sessions", distinct from the interactive default, or
3. At minimum: honor the documented settings-file model precedence for chip sessions, and document how app-managed worktree sessions resolve project settings.

Environment

• Claude Code Desktop on Windows 11 Pro (10.0.26200)
• Orchestrator session model: claude-fable-5; chip observed on claude-opus-4-8 then claude-fable-5
• Related but distinct: #78921 (per-call effort on Agent tool), #67794 (resumed subagent loses spawn-time model override)