feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/libs/markstream-vue/bin/prompts-lib.mjs

@@ -0,0 +1,49 @@
+import fs from 'node:fs/promises'
+import path from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url))
+export const packageRoot = path.resolve(__dirname, '..')
+export const defaultPromptsRoot = path.join(packageRoot, 'prompts')
+
+export async function listPromptFiles(promptsRoot = defaultPromptsRoot) {
+  const dirents = await fs.readdir(promptsRoot, { withFileTypes: true })
+  return dirents
+    .filter(dirent => dirent.isFile() && dirent.name.endsWith('.md'))
+    .map(dirent => dirent.name)
+    .sort()
+}
+
+export async function listPromptNames(promptsRoot = defaultPromptsRoot) {
+  return (await listPromptFiles(promptsRoot)).map(file => file.replace(/\.md$/i, ''))
+}
+
+export async function resolvePromptPath(name, promptsRoot = defaultPromptsRoot) {
+  const normalized = String(name ?? '').trim()
+  if (!normalized)
+    throw new Error('Prompt name is required')
+
+  const candidates = normalized.endsWith('.md')
+    ? [normalized]
+    : [`${normalized}.md`, normalized]
+
+  for (const candidate of candidates) {
+    const fullPath = path.join(promptsRoot, candidate)
+    try {
+      await fs.access(fullPath)
+      return fullPath
+    }
+    catch {
+      // continue
+    }
+  }
+
+  const available = await listPromptNames(promptsRoot)
+  throw new Error(`Unknown prompt: ${normalized}. Available prompts: ${available.join(', ')}`)
+}
+
+export async function readPrompt(name, promptsRoot = defaultPromptsRoot) {
+  const fullPath = await resolvePromptPath(name, promptsRoot)
+  const content = await fs.readFile(fullPath, 'utf8')
+  return { fullPath, content }
+}