[Mechanics] Add README section for mechanics

[asmelko]

Summary
Document mechanics subsystem usage and entry points in top-level README.

Steps

1. Edit README.md adding Mechanics section.
2. Describe environment role, agent flow, serialization output.
3. Provide minimal code snippet showing setup and loop.
4. Mention enabling serialization and sample output path.
5. Keep concise (< ~60 lines including snippet).

Acceptance Criteria

• README updated; section clear to new intern.

Verification
Render Markdown; ensure snippet compiles conceptually.

Dependencies
Mechanics integrated.

Follow-ups
Design diagram issue.