chore(repo): drop .NET implementation in favor of npm-only

The Node port (@kuns/claude-mailbox) is the recommended runtime and
covers all supported features. Maintaining a wire-compatible .NET twin
adds dual-impl tax with no remaining users, so remove src/, tests/,
solution + MSBuild files, NuGet config, and both .NET CI workflows.

Docs updated: README "Path C" build-from-source section dropped,
architecture diagram + Development section simplified, and node/README
no longer subtitles itself as a port of the .NET daemon.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/release-node.yml

@@ -85,7 +85,7 @@ jobs:
           TOKEN: ${{ secrets.GITEA_TOKEN }}
         run: |
           set -euo pipefail
-          # Try to find an existing release for this tag (the .NET workflow may have created it).
+          # Try to find an existing release for this tag (idempotent re-runs).
           EXISTING=$(curl -sS \
             -H "Authorization: token ${TOKEN}" \
             "${GITEA_API}/repos/${REPO}/releases/tags/${TAG}" || echo "")