Allow configuring plan file storage location (project-level option)

Problem

Plan files created via /plan mode are always saved to ~/.claude/plans/, the user-level directory. For solo developers or single-project workflows, this creates friction:

1. Plans are logically part of the project but stored outside the project directory, making them hard to discover and reference.
2. The user-level path (e.g., C:\Users\<username>\.claude\plans\) is long and unintuitive compared to a project-relative location.
3. When reviewing project context, plans are not visible alongside the code and documentation they relate to.
4. The current design optimizes for multi-user/multi-project separation, but many users work solo on a single project where that separation adds no value.

Proposed Solution

Add a configuration option (e.g., in .claude/settings.local.json or project-level settings) to specify the plan storage directory:

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

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

The default behavior (user-level storage) would remain unchanged for backward compatibility.

Use Case

• Solo developers managing a single project who want all Claude Code artifacts (commands, rules, specs, and plans) colocated within the project directory.
• Teams that want to commit finalized plans alongside design documents in the repo.

Environment

• Windows 11, Claude Code CLI
• Single-user, single-project workflow