Feature Request: Add plansDirectory setting for project-local plan storage

Summary

Add a plansDirectory setting to allow plans to be stored in a project-local .claude folder instead of the global ~/.claude/plans directory.

Use Case

When working on multiple projects, it would be useful to keep plan files alongside the project they relate to. This enables:

1. Version control - Plans can be committed to the repo for team visibility
2. Project isolation - Each project's plans stay with that project
3. Portability - Plans move with the project when cloning/moving repos
4. Cleanup - Deleting a project automatically cleans up its plans

Proposed Solution

Add a plansDirectory setting to the settings schema:

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

• Path would be relative to project root
• Default behavior unchanged (~/.claude/plans)
• Could be set in .claude/settings.json (shared) or .claude/settings.local.json (personal)

Alternatives Considered

• Symlinking ~/.claude/plans to project directories (manual, error-prone)
• Manually copying plans between locations (tedious)

Additional Context

This would align with the existing pattern of project-local .claude/ folders for settings and other project-specific configuration.