Configuration system and REST API

[rkuester]

Reviewing the YAML-configuration PR (#36) on dev call #2 made it clear we should pin down what we want from configuration before reviewing an implementation in detail. That writeup became the first Mujina Improvement Proposal (MIP).

MIP-0001 covers the configuration system and the REST API: one tree of named nodes unifying defaults, config files, environment variables, command-line arguments, and the API, with values that are readable, subscribable, validated, and persisted across restarts.

The document and its review live on the PR, so please read and comment there rather than here:

256foundation/mujina-mips#1

This also kicks off the MIP convention: when an idea grows into something that needs a written, editable design doc, it becomes a MIP in the mujina-mips repo, always introduced here first, so you can follow everything from Discussions and trust nothing appears there unannounced. Normally the discussion comes first and the idea takes shape there before any MIP exists; this thread is the exception, opening with a finished MIP because the document already existed. It subsumes the earlier static config file discussion (#23).