fix: use PATH-aware resolution for provider availability and fix Windows cmd escaping

Provider isAvailable() was using executableExists() which only checks
filesystem paths, not PATH. Commands like ["claude", "--flag"] would
show as unavailable even though they'd launch fine. Switch to
isCommandAvailable() which uses findExecutable() for proper PATH
resolution. Un-export executableExists from the public API.

Fix Windows cmd.exe metacharacter escaping — &, |, ^, <, >, (, ), !
are now properly escaped with ^, and % is doubled. Add shared
escapeWindowsCmdValue helper used by both quoteWindowsCommand and
quoteWindowsArgument. Replace local quoteForCmd in spawn.ts.

Add server-tests-windows CI job to catch Windows-specific regressions.

.github/workflows/ci.yml

@@ -76,6 +76,36 @@ jobs:
           CLAUDE_CODE_OAUTH_TOKEN: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
           OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
 
+  server-tests-windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+
+      - name: Fetch origin/main (worktree tests)
+        run: git fetch --no-tags origin main:refs/remotes/origin/main
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Build highlight dependency
+        run: npm run build --workspace=@getpaseo/highlight
+
+      - name: Build relay dependency
+        run: npm run build --workspace=@getpaseo/relay
+
+      - name: Run server tests
+        run: npm run test --workspace=@getpaseo/server
+        env:
+          CLAUDE_CODE_OAUTH_TOKEN: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
+          OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
+
   app-tests:
     runs-on: ubuntu-latest
     steps: