Auto-generate docs/reference.md from the CLI + config schema

## Problem

`docs/reference.md` (the CLI + config reference) is hand-maintained — it was copied out of the old README and is kept in sync by hand. That drifts: a new `mechababs` subcommand, a new flag, or a changed pipeline/cluster YAML field won't be reflected until someone remembers to edit the prose.

## Proposal

Generate the reference from the code that defines it:

- the CLI surface from the `argparse` definitions (subcommands, flags, help text);
- the config-file shapes from the pipeline / cluster YAML schema.

So the reference can't drift from the code, and adding a flag updates the docs by construction.

## Notes

- Keep the conceptual docs (`overview.md`, `interventions.md`) hand-written — this is only about the mechanical reference.
- Open question: generate at build/commit time (checked-in, reviewable diff) vs. on demand.

Start with docs/reference.md, the argparse definitions, and the pipeline and cluster YAML schema. Compare the current reference with those sources, then resolve whether generation runs at build/commit time or on demand. Done means the mechanical CLI and config reference is generated from code, remains reviewable, and leaves overview.md and interventions.md hand-written.

Written by the indexing model from the issue text.