[FEATURE] Project-specific plan storage location

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Description

Add a setting to store Plan Mode plans in the project's .claude/plans/ folder
instead of the global ~/.claude/plans/ folder.

### Use Case
When working on multiple projects, all plans are mixed in a single global folder,
making it difficult to:

• Keep project-specific plans organized
• Version control plans with the project
• Share plans with collaborators via git

Proposed Solution

### Proposed Solution
Add a setting in .claude/settings.json:
```json
{
"planStorage": "project"
}

When set to "project", plans save to <project_root>/.claude/plans/
instead of ~/.claude/plans/.

Benefits

• Better multi-project organization
• Plans can be version controlled
• Team members can share/review plans
• Consistent with project-level settings pattern

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

File operations

Use Case Example

_No response_

Additional Context

_No response_