docs: add plans/ directory for design and implementation plans

Introduce a new top-level `plans/` directory to host design and
implementation plans for larger refactoring efforts.

These documents capture the rationale, architecture, and step-by-step
implementation strategy for non-trivial changes before code is written.

Many of these plans are developed with the assistance of AI tools
(e.g. ChatGPT for prompt refinement/review and Claude for code analysis
and plan drafting), and then reviewed/edited by maintainers.

The goal is to:
- make design intent explicit and reviewable
- reduce iteration during implementation
- provide a reference for future contributors
- allow the flexibility of having one person draft a plan
  and pass it on to another to execute it.

No functional changes.

Signed-off-by: Martin Belanger <[email protected]>

Author: Martin Belanger