docs: align protocol and session docs with dev

[Leon-Dicaprio]

Summary

• Add docs/LINGOW_PROTOCOL_DESIGN.md for the current Lingow protocol design.
• Align the protocol design with the latest dev contracts.
• Update services/api/sessions/README.md so it reflects the current Sessions implementation, public routes, realtime adapter ownership mapping, production composition, and the remaining Runtime Failure transport boundary.

Validation

• git fetch --all --prune
• git rebase upstream/dev
• git diff --check
• README stale-text grep for outdated implementation-status language

Notes

This PR targets 1024XEngineer/xe6-tsy:dev from Leon-Dicaprio/xe6-tsy:docs/p0-protocol-design. The local working tree still has an untracked .worktrees/ directory that was not included.

[github-actions]

Go coverage

Scope	Current	Baseline	Delta
packages/contracts	84.2%	84.2%	+0.0%
services/api	55.7%	55.6%	+0.0%
services/realtime-audio	78.4%	78.4%	+0.0%
Total	64.4%	64.4%	+0.0%

Base branch: dev · Commit: 31f8c2b · Test workflow run

[fennoai]

Review Summary

Found three protocol inconsistencies that could cause incompatible clients or spoofed actor metadata. See the inline findings.

View job run

[Jerry-Yu-TeslaCoil]

这是纯文档 PR（协议设计 + sessions README），CI 全绿。对照过现有 contracts/实现后，早期 bot 提的字段不一致（session_id、started_by 身份约束、speaker_label_snapshot）在当前 HEAD 已对齐，没发现会误导客户端或造成错误实现的大问题。