v1.3.0: 4 new flavors, Claude Code chip fix, 2026 surfaces, new visuals

[ashwingopalsamy]

Closes #2. Prepares #3 (Open VSX), which goes live once the OVSX_PAT secret is set.

• New flavors: Midnight (OLED), Dusk (claude.ai dark), Mono Dark, Mono Light. Each has a (No Bold) twin, for 20 themes total. Existing theme labels and paths are unchanged.
• [BUG] High Contrast reference color #2: Claude Code's @-file chip reads chat.slashCommandBackground/Foreground. Both are now an opaque clay pair with 5.8–10:1 contrast, and ≥7:1 in High Contrast. textPreformat.* gets the same pair.
• Fix: Brand (No Bold) themes were missing the brand chrome, because of a variant.endsWith('brand') check.
• Coverage: 414 → 610 workbench keys per theme, covering agent sessions, inline edits, SCM graph, terminal symbol icons, sticky scroll, gauges, markdown alerts and the modern UI keys.
• Generator: Flavor is now separate from weight. build.ts is the single source for the theme files and package.json contributes.themes.
• Visuals: pnpm shots renders all screenshots from the theme JSONs using Shiki and Playwright. Assets are excluded from the VSIX.
• Docs: README rewritten (61 lines), new CHANGELOG, internals moved to CONTRIBUTING.

After merge, tag v1.3.0 to publish.