fix(tui): preserve system palette colors

[opencode-agent]

Summary

• generate a native V2 system theme directly from the detected terminal palette in system.ts
• preserve literal ANSI hues across V2 hue steps instead of synthesizing darker colors
• share palette resolution with the temporary V1 output used by remaining legacy surfaces; no system-theme migration path
• inherit the normal V2 semantic token defaults from the generated hue scales

Before / after

The generic V1 migration synthesized darker shades that do not exist in the terminal palette. With a Tokyo Night terminal palette:

• Agent purple: #a485db → #bb9af7
• Accent cyan: #306a8a → #7dcfff

OpenCode Drive before/after/pixel-diff comparison

The visual comparison uses the real TUI at 100×30 with a generated Tokyo Night system palette seeded through the normal custom-theme loader. OpenCode Drive's simulated terminal does not answer OSC palette queries, so direct native-system generation is additionally covered by test/theme/system.test.ts.

Checks

• bun test test/theme/system.test.ts test/theme/v2/v1-migrate.test.ts
• bun typecheck (from packages/tui)
• git diff --check

Requested by: @kitlangton (Kit Langton via Slack)

[opencode-agent]

Reworked this around the native seam: system.ts now generates the V2 theme directly from terminal colors and inherits V2 semantic defaults. migrateSystem is gone; V1 output remains only for legacy surfaces.