feat(plugins): Theme plugin template, theme docs and IntelliJ theme import (#54)

Per the maintainer's decision for #24, no third-party themes are bundled; instead a theme plugin template and docs let the community port themes (IntelliJ also ships few themes, the rest are plugins).
- templates/theme-plugin: manifest-only plugin with an original example (Harbor Night) covering every UI token and an editor scheme (.icls equivalent).
- docs/plugins/themes.md: format, IntelliJ / VS Code porting tables, generated token reference (pnpm gen:theme-docs); drift test fails when tokens.css and the template/reference disagree.
- pnpm theme:import-intellij: dependency-free converter for IntelliJ .theme.json (+ .icls).
- Plugin host: themes may name an editor-scheme merged over the theme; comments allowed in theme files.
- Fixes: a plugin theme's kind now switches the light/dark base; plugin themes go in their own stylesheet so the Appearance accent and editor scheme still layer on top.
Verified: ide-plugin tests, vitest (44), in-app on macOS: installed the template from disk, applied Harbor Night (dark base + teal accent over Islands Light), back to default restored the light theme.

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

plugins/README.md

@@ -179,8 +179,14 @@ never affected.
 
 Tokens are the CSS variables of `src/styles/tokens.css` (`--bg-*`, `--fg-*`,
 `--border*`, `--accent`, `--syn-*`, `--vcs-*`, `--gutter-*`, …). Unlisted tokens
-keep the built-in values. Values must be plain CSS values (no `url()`, `;`,
-braces). Pick a theme in Settings › Plugins › *Apply*.
+keep the values of the built-in base named by `kind`. Values must be plain CSS
+values (no `url()`, `;`, braces); `//` and `/* */` comments are allowed. A
+manifest theme entry may add `editor-scheme = "themes/x.scheme.json"` (editor
+colours, merged over the theme). Pick a theme in Settings › Plugins › *Apply*.
+
+Start from [`templates/theme-plugin`](../templates/theme-plugin); the full guide,
+token reference and IntelliJ / VS Code porting tables are in
+[`docs/plugins/themes.md`](../docs/plugins/themes.md).
 
 ## 4. Languages