Add README to plans/ folder

Context

The repository has a top-level plans/ directory with development planning artifacts organized by issue or initiative, for example plans/343/..., plans/645/..., and named workstream folders. There is not currently a top-level plans/README.md explaining what these files are or how humans and coding agents should use them.

Without that context, the folder can be mistaken for product documentation, architecture source of truth, or scratch space, and agents may add or update plans inconsistently.

Proposal

Add plans/README.md that explains:

• what the planning docs are and how they relate to GitHub issues, PRs, and implementation work
• the expected folder naming conventions, such as plans/<issue-number>/... for issue-backed work and named workstream folders when appropriate
• what belongs in plans/ versus product docs, architecture/, or temporary scratch notes
• guidance for agents: keep plans factual, link relevant issues and PRs, update stale plans when work changes, and avoid treating plans as user-facing documentation
• how to include supporting assets such as diagrams or images when a plan needs them

Acceptance criteria

• A top-level plans/README.md exists.
• The README gives concise context for both human contributors and coding agents.
• It documents the current observed structure of the plans/ directory.
• It clarifies that these are development planning artifacts, not product documentation.
• It includes enough guidance that future plans can be added consistently.

Start by inspecting the existing plans/ directory, including plans/343/ and plans/645/ and named workstream folders, to document its current structure accurately. Create plans/README.md covering the proposal and acceptance criteria, then verify that it distinguishes planning artifacts from product documentation, architecture/, and scratch notes and gives consistent guidance for humans and coding agents.