Add dual-lane steering and follow-up timeline

[tmustier]

Summary

• show steering and follow-ups as independent lanes in one delivery-ordered timeline
• keep steering editable until Pi’s native turn boundary and feed follow-ups into Pi’s native continuation queue
• add rollback-safe cross-lane editing, recency-first selection, Option+Down navigation and pinning
• pause both lanes after abort and require explicit resume
• honour Pi’s one-at-a-time and all modes at active-run boundaries
• replace the v0.1.0 capture with a new dual-lane demo

Verification

• npm run ci (28 tests)
• git diff --check
• live Pi 0.80.9 proof with real tool turns: steering arrived at the next boundary and follow-ups arrived after the run
• live rollback proof across 3 edited rows
• live abort, pause and resume proof
• live all-mode proof for both lanes
• visual compatibility proof with pi-session-hud and raw-paste
• dual-lane demo visually reviewed at 1× and 2× speed

Before release

• solve or accept the documented edge case where an all-mode lane remains pinned until the agent settles; idle resume sends the head, then releases the remainder at the next boundary