docs: add Qwen Code install instructions to README

Rebased from PR #2108 onto the reworked README. Differences from the PR:
the Hermes TOC entry and Quickstart-line changes are obsolete (the v6.3.0
README rework already added the former and removed the latter), and the
Hermes post-compaction caveat stays: .hermes-plugin injects the bootstrap
only on is_first_turn, so the caveat is still accurate.

Install/update commands and the acceptance transcript are from PR #2108
(@arittr, tested interactively on Qwen Code).

Co-authored-by: Drew Ritter <arittr@users.noreply.github.com>

README.md

@@ -20,6 +20,7 @@ Superpowers is a complete software development methodology for your coding agent
   - [Kimi Code](#kimi-code)
   - [OpenCode](#opencode)
   - [Pi](#pi)
+  - [Qwen Code](#qwen-code)
   - [Hermes Agent](#hermes-agent)
 - [The Basic Workflow](#the-basic-workflow)
 - [Community](#community)
@@ -246,6 +247,22 @@ pi -e /path/to/superpowers
 
 The Pi package loads the Superpowers skills and a small extension that injects the `using-superpowers` bootstrap at session startup and again after compaction. Pi has native skills, so no compatibility `Skill` tool is required. Subagent and task-list tools remain optional Pi companion packages.
 
+### Qwen Code
+
+Qwen Code installs plugins from Claude Code marketplaces directly.
+
+- Install the plugin from this repository, and pick `superpowers` when prompted:
+
+  ```bash
+  qwen extensions install obra/superpowers
+  ```
+
+- Update later:
+
+  ```bash
+  qwen extensions update superpowers
+  ```
+
 ### Hermes Agent
 
 Install Superpowers as a Hermes plugin from this repository: