feat: real channel-bridge client + reply tool

- WS client to OpenclawPlugin's bridge (auto-reconnect with exponential
  backoff)
- On hello: send (openclaw_session, claude_session, pid) — no auth token
- On inbound frame: emit notifications/claude/channel into MCP so Claude
  Code opens a new turn
- Single MCP tool `reply(text, final?)` — model calls it to send a
  reply back via the bridge. final=false streams progress chunks
- Capability declared under `experimental.claude/channel` (top-level was
  silently ignored — bug confirmed against Anthropic's discord plugin)
- Dropped auth handshake + permission_request reverse channel per the
  same-machine, full-perms design call

Add .mcp.json so the package is also a valid Claude Code plugin (for
potential plugin: form deployment later).

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "synthesis": {
+      "command": "bun",
+      "args": ["run", "--cwd", "${CLAUDE_PLUGIN_ROOT}", "--shell=bun", "--silent", "start"]
+    }
+  }
+}