docs: add AGENTS.md with agent conventions

[andig]

Adds an AGENTS.md so AI coding agents have one source of conventions for this repo, modeled on evcc-io/evcc's AGENTS.md and adapted to the optimizer's Python MILP plus Go client stack.

• Documents the make based workflow (make test, make lint, make run, make build) and the uv run requirement
• Captures the MILP domain rules: maximization with negative penalties, Wh and W units, the positive penalty floor, and that charging strategies are cost-neutral tie-breakers excluded from the clean objective
• Notes the get_clean_objective_value() first step baseline that can surprise scenario authors
• States that openapi.yaml is the API source of truth and client/client.gen.go is generated, never edited by hand
• Adopts the evcc writing style, comment style, and PR description rules