Support per-project agent auto-loading via settings.json

Feature Request

Is your feature request related to a problem?

When using custom agents (via --agent), there's no way to auto-load a specific agent for a project. I have to remember to launch with claude --agent my-agent every time I open a session in that project directory. If I forget, I get a vanilla session without my agent's persona.

Describe the solution you'd like

A new agent key in .claude/settings.local.json (or settings.json) that auto-loads the specified agent on session start:

{
  "agent": "my-agent"
}

This would be equivalent to launching with claude --agent my-agent but tied to the project directory.

Describe alternatives you've considered

• SessionStart hook that cats the agent file — injects context but doesn't replicate full --agent behavior
• Shell aliases — work but require remembering to use the alias instead of claude
• Launch scripts — same issue, easy to forget

Additional context

This would make project-specific agents much more seamless — open a terminal in the project dir, run claude, and the right agent loads automatically.