feat(backends): onboard OpenCLI as cross-channel desktop backend

- new agent_reach/backends/opencli.py: probes install + daemon/extension
  state via `opencli daemon status` (pure query — `opencli doctor`
  auto-starts the daemon, a side effect health checks must avoid)
- `agent-reach install --channels opencli`: npm install + Chrome Web
  Store guide (extension install cannot be automated — Chrome security
  model — so we print the one-click path)
- server env skips OpenCLI (rides a real desktop Chrome session)
- channels will adopt it as a backend candidate in follow-up PRs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent_reach/backends/__init__.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+"""Cross-channel backends.
+
+A backend here is an upstream runtime that serves MULTIPLE channels
+(e.g. OpenCLI covers xiaohongshu/reddit/bilibili/twitter through one
+browser session), as opposed to the per-platform tools probed inside
+each channel file.
+"""
+
+from .opencli import (  # noqa: F401
+    OPENCLI_EXTENSION_URL,
+    OPENCLI_PACKAGE,
+    OpenCLIStatus,
+    opencli_status,
+    opencli_summary,
+)