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/xiaomi-tts/SKILL.md

@@ -118,7 +118,7 @@ Generate speech via media-proxy's `/chat/completions` endpoint.
 **Important**: `messages` must use the `assistant` role (not `user`); the text to synthesize goes in the assistant message's content.
 
 ```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 '{
@@ -173,7 +173,7 @@ The audio comes back as base64; decode it and save to the local media-store.
 **Recommended approach** (write the full response to a file first to avoid overlong shell arguments):
 
 ```bash
-PORT=$(cat ~/.desirecore/agent-service.port)
+PORT=$(cat ${DESIRECORE_ROOT}/agent-service.port)
 # Save the full request and response to a file
 curl -sk -X POST "https://127.0.0.1:${PORT}/api/media-proxy" \
   -H "Content-Type: application/json" \

skills/xiaomi-tts/SKILL.zh-CN.md

@@ -54,7 +54,7 @@
 **重要**：messages 必须使用 `assistant` role（不是 user），要合成的文本放在 assistant 消息的 content 中。
 
 ```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 '{
@@ -109,7 +109,7 @@ curl -sk -X POST "https://127.0.0.1:${PORT}/api/media-proxy" \
 **推荐方式**（先保存完整响应到文件，避免 shell 参数过长）：
 
 ```bash
-PORT=$(cat ~/.desirecore/agent-service.port)
+PORT=$(cat ${DESIRECORE_ROOT}/agent-service.port)
 # 将完整请求和响应保存到文件
 curl -sk -X POST "https://127.0.0.1:${PORT}/api/media-proxy" \
   -H "Content-Type: application/json" \