docs(readme): align documentation with current dev runtime

[Gwen317]

Summary

• Rewrite the root README to describe the current dev runtime, control/media split, assistant and interpretation modes, delivery behavior, device SDK, and current boundaries.
• Replace the obsolete root pnpm startup commands with the supported Windows launcher, Docker Compose, Go service, Web, and Mobile commands.
• Align the Coverage badge block with main so the existing README merge conflict is resolved without duplicate badges.

Verification

• git diff --check
• Three-way merge simulation against upstream/main: no conflicts in the resulting tree.
• Merged README contains one Coverage badge and no conflict markers.

[github-actions]

Go coverage

Scope	Current	Baseline	Delta
packages/contracts	90.2%	90.2%	+0.0%
services/api	63.6%	63.6%	+0.0%
services/realtime-audio	79.2%	79.3%	-0.0%
Total	71.5%	71.5%	-0.0%

Files below baseline

Showing the 5 largest file-level regressions.

File	Current	Baseline	Delta
services/realtime-audio/github.com/1024XEngineer/xe6-tsy/services/realtime-audio/pipeline/flow.go	84.0%	84.6%	-0.6%

Base branch: dev · Commit: 4c725ff · Test workflow run

[fennoai]

已审阅固定范围 fdf669e00ac72e5c72d5cd767c124bd7c670cc46...2242cc4403c7d6e6c11a67372e4c75854e54a684 中的 README.md。文档新增的运行边界、环境变量、启动命令、包脚本、基础设施版本和内部链接均与当前仓库内容一致，未发现需要作者修复的正确性、兼容性或可维护性问题。

验证：

• git diff --check 通过，README 无冲突标记且仅保留一个 Coverage badge。
• README 中列出的本地链接均存在。
• go test ./packages/contracts/... ./services/api/... ./services/realtime-audio/... ./sdks/device/... 通过。