revert(tasks-ui): drop the shadcn/Tailwind layer and return to the plugin's own UI system

shadcn's defaults are built for a standalone app: 14px body, 36px controls,
solid primary fills and elevation read as a foreign surface next to this
plugin's own tabs (files, changes, side chat, settings), which are 12/11/10px
type, 28px controls, hairline borders and flat panels. Retuning the layer did
not converge, so the layer goes.

- Removed: src/client/ui/** (21 vendored components), components.json,
  scripts/ui-css.mjs, the Tailwind/PostCSS compile step in tsdown, and the deps
  tailwindcss / @tailwindcss/postcss / postcss / radix-ui /
  class-variance-authority / tailwind-merge. (diff and simple-icons stay: they
  patch undeclared bare imports of the host primitives, unrelated to shadcn.)
- The seven page surfaces are back on the plugin's own base: *.module.css with
  --dsw-* tokens plus the host primitives (Button/Menu/Modal/Pill/Tag/Switch/
  Input/Tooltip/StateDot/MarkdownText). Behaviour, data-* hooks, aria labels,
  i18n keys and the AnchoredPopover geometry/drag/dismiss contract are
  unchanged; the non-styling fixes landed during the shadcn era (model id
  dedupe, task-window CAS, drawer arming, popover dragging, the chunk packaging
  glob) were re-applied and are covered by the existing specs.
- The tasks page returns to the core bundle: `tasks` left CHUNK_NAMES /
  CHUNKS / ChunkName, src/client/chunks/tasks.tsx is gone and tabs.tsx imports
  SubagentView directly.
- The reverted stylesheet was retuned to the plugin's measured vocabulary
  (12/11/10px, 28px controls, border-l2 with border-l1 hairlines, hover fills,
  no static shadows, --dsw-shadow-lv2/3 on floating layers only) and verified
  against a 70-assertion real-browser contract; the harness's own two
  measurement defects were fixed with a falsification test proving the
  assertions still catch injected regressions (56/70, 14 FAIL).
- Tests: ui-foundation / ui-bundle / ui-shadows (the shadcn guards) deleted,
  theme.spec scoped back to the plugin's own stylesheets, bundle-route and
  manifest-consistency chunk lists back to four. Docs: AGENTS.md and the plugin
  guide drop the Tailwind/shadcn guidance; the design doc marks those sections
  superseded and records why the layer was removed.

Verified: typecheck/lint clean; 133 files / 1410 tests; core bundle 987.3 KiB
with no radix-ui/--tw-/tailwind strings and no client-tasks.js; mount e2e 7/7;
20 real-browser screenshots at 360/720 for graph, tree, board, task window,
drawer (including armed), both popovers and the 77-node fixture.

tests/theme.spec.ts

@@ -14,7 +14,7 @@
 // @vitest-environment jsdom
 import { afterEach, describe, expect, it } from 'vitest'
 import { readFileSync, readdirSync } from 'node:fs'
-import { resolve } from 'node:path'
+import { resolve, sep } from 'node:path'
 import { colorAlpha, effectiveTokenValue, tokenValue } from '../src/client/theme.ts'
 
 afterEach(() => {
@@ -91,12 +91,31 @@ describe('effectiveTokenValue', () => {
  * `FileTypeIcon` artwork from a platform module (the host owns those pixels
  * and its own palette), and everything the plugin renders around them —
  * including the colored tab glyphs — rides theme tokens. So the guard is
- * simply that no plugin module carries a color literal, and that no icon
- * dataset sneaked back in as a chunk.
+ * simply that no plugin module carries a color literal, that every module
+ * stylesheet paints `color` from a token, and that no icon dataset sneaked
+ * back in as a chunk.
  */
 // jsdom has no file:// import.meta.url; vitest runs from the repo root.
 const ROOT = process.cwd()
 
+/** `color` as a property — never `background-color` / `scrollbar-color` / `-webkit-text-fill-color`. */
+const COLOR_PROPERTY = /(?:^|[;{\s])color\s*:\s*([^;}]+)/g
+
+/** Values that name no paint of their own and are therefore exempt. */
+const INERT_COLOR = /^(?:inherit|currentcolor|transparent)$/i
+
+/** A token allowed by the contract: the plugin's own `--dsw-*` or the host's `--ds-*`. */
+function isThemeToken(name: string): boolean {
+  return name.startsWith('dsw-') || name.startsWith('ds-')
+}
+
+/** Every `*.module.css` sheet under `src/client/`, as repo-relative POSIX paths. */
+function moduleStylesheets(): string[] {
+  return readdirSync(resolve(ROOT, 'src/client'), { recursive: true, encoding: 'utf8' })
+    .filter(name => name.endsWith('.module.css'))
+    .map(name => `src/client/${name.split(sep).join('/')}`)
+}
+
 describe('skin contract: the plugin owns no color of its own', () => {
   it('the icon modules carry no color literals', () => {
     for (const file of ['src/client/file-icons.tsx', 'src/client/icons.tsx']) {
@@ -121,6 +140,27 @@ describe('skin contract: the plugin owns no color of its own', () => {
     }
   })
 
+  it('every module stylesheet paints `color` from a theme token', () => {
+    const sheets = moduleStylesheets()
+    // Guard the scan itself: a glob that silently matches nothing would make
+    // this contract vacuous.
+    expect(sheets.length).toBeGreaterThan(5)
+    for (const file of sheets) {
+      const styles = readFileSync(resolve(ROOT, file), 'utf8').replace(/\/\*[\s\S]*?\*\//g, '')
+      for (const declaration of styles.matchAll(COLOR_PROPERTY)) {
+        const value = declaration[1]!.trim()
+        // `color: inherit` / `currentcolor` / `transparent` is the plugin
+        // deliberately taking the surrounding color, not painting one.
+        if (INERT_COLOR.test(value)) continue
+        // Dereference every var() — `color-mix()` chains are fine as long as
+        // the whole expression bottoms out in theme tokens.
+        const tokens = [...value.matchAll(/var\(--([a-z0-9-]+)/gi)].map(match => match[1]!)
+        expect(tokens.length, `${file}: color: ${value}`).toBeGreaterThan(0)
+        for (const name of tokens) expect(isThemeToken(name), `${file}: color: ${value}`).toBe(true)
+      }
+    }
+  })
+
   it('the empty-pane welcome capsule follows the host guide recipe', () => {
     // The card is the same surface as DSH's own guide capsule ("pick what
     // opens here"), so this pins the shared geometry and the token-only