feat: Add model parameter to spawn session options

[mark9232]

Summary

Allows mobile app to specify model (e.g., 'sonnet', 'opus') when spawning sessions.

Changes:

• Add model field to SpawnSessionOptions interface
• Extract model from spawn-happy-session RPC params
• Set ANTHROPIC_MODEL env var from model option (takes precedence over profile)
• Pass --model flag to Claude CLI when spawning (both regular and tmux modes)

Problem

When selecting a model in the Happy Coder mobile app before spawning a session, the model selection was ignored. Sessions always started with the default model (Opus 4.5) regardless of user selection.

Evidence from daemon logs:

[API MACHINE] Spawning session with params: {"type":"spawn-in-directory","directory":"...","agent":"claude"}

No model field was being passed or used.

Solution

This PR adds support for a model field in the spawn session options. When the mobile app sends model: "sonnet", the daemon will:

1. Set ANTHROPIC_MODEL=sonnet in the spawned process environment
2. Pass --model sonnet flag to the Claude CLI

The fix is backward compatible - if mobile doesn't send model, behavior is unchanged.

Files Changed

• src/modules/common/registerCommonHandlers.ts - Add model to interface
• src/api/apiMachine.ts - Extract and pass model from RPC params
• src/daemon/run.ts - Use model in env and CLI args

Testing

• Spawn session from mobile with model selection
• Verify daemon logs show model being set
• Verify Claude uses the correct model

Note for Mobile App

The mobile app (slopus/happy) needs to be updated to send model in the spawn params:

{
  "type": "spawn-in-directory",
  "directory": "/path/to/dir",
  "agent": "claude",
  "model": "sonnet"
}

Fixes #158

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Co-Authored-By: Claude noreply@anthropic.com
Co-Authored-By: Happy yesreply@happy.engineering