[Feature Request] Project-local plans directory

Currently, plan files are stored in ~/.claude/plans, which is global to all projects. I'd like the ability to store plans in a project-local directory so that:

1. Plans stay with the project - They can be committed to git, shared with team members, and persist when the project is cloned elsewhere
2. Project isolation - Different projects don't pollute each other's plan namespace
3. Portability - Plans move with the codebase, not tied to a specific machine

Suggested implementation:

Add a planDir setting in .claude/settings.json:

{
    "planDir": ".claude/plans"
}

When set, plans would be stored relative to the project root instead of ~/.claude/plans

Workarounds considered:

• Symlinking ~/.claude/plans to the project dir: breaks when switching projects
• CLAUDE_CONFIG_DIR env var: moves all config, not just plans

Neither is ideal for multi-project workflows.