Add Plan Command workflow for breaking down issues into sub-tasks

[Copilot]

Adds the /plan command workflow from githubnext/gh-aw to enable automatic breakdown of issues and discussions into actionable sub-tasks.

Changes

• workflows/plan.md: Plan Command workflow that responds to /plan comments on issues/discussions, creates up to 5 focused sub-issues with objectives, context, and acceptance criteria
• docs/plan.md: Documentation covering usage, configuration, and best practices
• README.md: Added Plan Command to Research, Status & Planning Workflows section

Usage

Comment /plan on any issue or discussion to generate sub-tasks:

Issue: "Implement user authentication system"
→ /plan
→ Creates:
  [task] Add JWT authentication middleware
  [task] Create user login endpoint
  [task] Implement password hashing
  [task] Add authentication tests
  [task] Update API documentation

The workflow automatically labels created issues with task and ai-generated, and closes "Ideas" discussions after planning.

Original prompt

Add a copy of the "/plan" agentic workflow in githubnext/gh-aw. Make it generic.

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