Add opt-in `inject_sender` config option for projects. When enabled,
cc-connect prepends a metadata line to each message before forwarding
to the agent:
[cc-connect sender_id=ou_abc123 platform=feishu]
<original message>
This allows agents (Claude Code, Codex, etc.) to identify who sent the
message and adjust behavior accordingly — e.g. personal task views,
role-based access control, or per-user tone.
The feature is off by default and can be enabled per-project:
[[projects]]
inject_sender = true
Closes#91