fix: replace hardcoded ~/.desirecore paths with ${DESIRECORE_ROOT} variable (#16)

## Summary

- 将所有技能文件中的硬编码 `~/.desirecore/` 和 `$HOME/.desirecore/` 路径替换为
`${DESIRECORE_ROOT}/` 变量
- 递增 manifest.json version 至 1.2.1

## Why

dev 模式下 `DESIRECORE_HOME=~/.desirecore-dev`，硬编码路径导致技能读取错误的端口文件和目录。主仓库的
`variable-substitutor.ts` 会在运行时将 `${DESIRECORE_ROOT}` 替换为实际根目录。

## Test plan

- [ ] `npm run dev` 启动后触发任意技能，确认端口路径解析为
`~/.desirecore-dev/agent-service.port`
- [ ] prod 模式确认路径为 `~/.desirecore/agent-service.port`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

skills/dashscope-image-gen/SKILL.md

@@ -95,7 +95,7 @@ market:
 Generate the image via media-proxy's compatible-mode endpoint; the response includes the image URL directly:
 
 ```bash
-PORT=$(cat ~/.desirecore/agent-service.port)
+PORT=$(cat ${DESIRECORE_ROOT}/agent-service.port)
 curl -sk -X POST "https://127.0.0.1:${PORT}/api/media-proxy" \
   -H "Content-Type: application/json" \
   -d '{
@@ -151,7 +151,7 @@ Locate the item with `type: "image"` inside `data.output.choices[0].message.cont
 The image URL is time-limited; download and persist it to the local media-store immediately:
 
 ```bash
-PORT=$(cat ~/.desirecore/agent-service.port)
+PORT=$(cat ${DESIRECORE_ROOT}/agent-service.port)
 IMAGE_URL="image URL from step 1's response"
 curl -sL "$IMAGE_URL" -o /tmp/dashscope-gen.png && \
 curl -sk -X POST "https://127.0.0.1:${PORT}/api/media/upload" \