Session section, sub-agent roll-ups, and two-box sidebar

[charlesnutter]

The Session section, sub-agent roll-ups, and the sidebar redesign. Ships as 0.3.0.

What changes

• Two boxes in the sidebar, the last turn and the session, each opened and closed by clicking its heading. Labelled rows, one figure per line; the heading names the engine only. Shaded with the theme's background.raised.high (option background, default on), padded 1 row by 2 columns.
• Session section (collapsed by default): generation tok/s with a trend, TTFT median and max, cache hit, time split (generating / waiting / sub-agents / other), engine averages, retries. Current model's turns only.
• Sub-agent roll-ups: tokens, span and cost of a turn's sub-agents; never a combined rate. Same-engine sub-agents are expected in counter-difference windows and labelled incl. sub-agents.
• Latest-request engines (mtplx, koboldcpp, mlxserve) are read at step.streamed, before tools run, so a same-engine sub-agent no longer displaces the step's figures.
• History panel headline rate is tokens over streaming time on the newest model, not a mean of rates across models.

See CHANGELOG.md [Unreleased] for the full list.

Verified

• npm test and tsc --noEmit pass.
• Live on OpenCode 2.0.12 with MTPLX: both boxes, collapse, shading, padding, sub-agent roll-up, session figures.

Not verified live

• Same-engine sub-agent windows on counter-difference engines (captures only).
• KoboldCpp and mlx-serve per-step reads (fixtures only).

Parked

• History panel row wrapping: per-row truncation garbled rows on 2.0.12, reverted.
• Skipped-turn count in the Session section.