Dialog tests and README rework (0.5.0) - #7

Tests for the tabbed dialog, and the README rework. Ships as 0.5.0 with #6.

Tests

• test/e2e/dialog.e2e.mjs (6 scenarios): real turns through the entry file, then each tab laid out as the entry file lays it out -- Turn (time split adds up, timeline per step, tools, generated tokens, engine grid), every tab fitting at 72/80/106/130 cells, Session (tools by time, "no tool calls", coverage, tokens), History (this session / all), and the spacing rule.
• test/dialog.test.mjs (7): bar arithmetic, share rounding, aligned columns, bar give-way, column-grid fallback, and the long-legend-name gap.
• Both regression tests (the legend gap, "1 turns") fail on the pre-fix code.

Fixes

• History headline says 1 turn, not 1 turns.
• Removed the unused packRows.

README

• Screenshots of the sidebar and the dialog's three tabs (2x2, served from main's assets/; they render once this is merged).
• A features list, a shorter intro, and the OpenCode 2 requirement moved into Install.
• Important Notes cut to a few sentences per subsection; the full text kept in docs/measurement-notes.md, with the two points now out of date flagged at its top.

npm test and tsc --noEmit pass.