feat: migrate to OpenCode 2

OpenCode 2 publishes its CLI as @opencode/cli (opencode-ai stops at
1.19.x), so install that package and pin it to 2.0.32.

Also fix what the v2 migration invalidates:
- renovate.json: track @opencode/cli instead of opencode-ai, otherwise
  the pin is never bumped.
- opencode-attach: `opencode attach` no longer exists in v2. Point the
  client at the server with `--server <url>` instead (appended after a
  subcommand when one is given); auth still comes from
  OPENCODE_SERVER_PASSWORD / OPENCODE_PASSWORD.
- README/AGENTS: v2 package name, entrypoint, serve defaults, API
  surface (/global/health is gone, everything lives under authenticated
  /api/*), and removal of OPENCODE_SERVER_USERNAME (the username is
  always `opencode` in v2).

The entrypoint stays ["opencode"] on purpose: existing deployments pass
`serve ...` at runtime, and OpenCode 2 is invoked the same way.

renovate.json

@@ -12,12 +12,12 @@
     },
     {
       "customType": "regex",
-      "description": "Track opencode-ai npm package pinned in Dockerfile RUN command",
+      "description": "Track @opencode/cli npm package pinned in Dockerfile RUN command",
       "managerFilePatterns": ["/^Dockerfile$/"],
-      "matchStrings": ["opencode-ai@(?<currentValue>[^\\s]+)"],
-      "depNameTemplate": "opencode-ai",
+      "matchStrings": ["@opencode/cli@(?<currentValue>[^\\s]+)"],
+      "depNameTemplate": "@opencode/cli",
       "datasourceTemplate": "npm",
       "versioningTemplate": "npm"
     }
   ]
-}
+}