Refactor PLAN.md template management for better Copilot integration

[Copilot]

Addresses design feedback on #6809 regarding PLAN.md template organization and Copilot usability.

The current implementation embeds the PLAN.md template within .claude/commands/plan.md command documentation. This works but requires parsing markdown to extract the template programmatically.

Recommendation

Extract template to dedicated file:

• Create .claude/templates/plan-template.md with the canonical template structure
• Update .claude/commands/plan.md to reference the template file instead of embedding it
• Maintains current PLAN.md at repo root for testing cleanup workflow

Benefits

• Programmatic access: Read template directly without markdown parsing
• Single source of truth: Command docs reference template instead of duplicating content
• Consistency: Eliminates risk of template drift between documentation and usage
• Separation of concerns: Commands describe behavior, templates provide structure

Current template structure (from .claude/commands/plan.md lines 48-70):

# [Title]

## Source
- **Repo**: [org/repo](https://github.com/org/repo)
- **Issue**: [#123](https://github.com/org/repo/issues/123)
- **Branch**: `branch-name`
- **Created**: YYYY-MM-DD

## Objective
<!-- What are we trying to accomplish? -->

## Tasks
- [ ] Task 1
- [ ] Task 2

## Notes
<!-- Research, decisions, blockers -->

This refactoring improves Copilot's ability to programmatically create and manage planning documents while maintaining the cleanup workflow testing approach.

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