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.

opencode-attach

@@ -45,4 +45,12 @@ trap cleanup_lock INT TERM EXIT
 kill_existing_instance
 printf '%s\n' "$$" > "$LOCKFILE"
 
-exec opencode attach -p "${OPENCODE_SERVER_PASSWORD}" "${OPENCODE_API_URL}" "$@"
+# OpenCode 2 dropped `opencode attach`: a client is pointed at a server with
+# --server <url>, which must follow the subcommand when one is given.
+if [ "$#" -eq 0 ]; then
+    set -- --server "${OPENCODE_API_URL}"
+else
+    set -- "$@" --server "${OPENCODE_API_URL}"
+fi
+
+exec opencode "$@"