Clarify stable vs experimental commands

[TheNorthStarYang]

Goal

Clarify which protocol commands should be considered stable and which are still experimental.

Scope

• improve wording and structure in protocol docs
• keep the distinction between stable / experimental / internal clear
• avoid unrelated refactors

Where to edit

• repository: protocol
• relevant path: docs/stability-policy.md
• related files: docs/commands.md

Notes

This issue is part of the early public protocol cleanup.

Contributors are welcome.