con-duct plot each job after mechababs finishes

# con-duct plot each job after mechababs finishes

## Problem

Mechababs runs collect per-job duct logs. After a run finishes, it would be
useful to auto-generate `con-duct plot` outputs for each job — a quick visual
sanity check on resource use without chasing down logs manually.

## Context

Builds on con-duct's plot tooling. `con/duct#424` (plot-pcpu-fix) is **merged**,
so we can wire this in now. `con/duct#423` (pcpu-delta-spike) is in flight —
**not a blocker**, but would give better data once it lands.

## Next

- Decide where the plotting step lives: post-run finalize hook, a
`mechababs plot` subcommand, or an external script consuming the run dir.
- Pick a representative completed run as the prototype target.
- Wire it in (works against merged `con/duct#424`; revisit for `con/duct#423`).