feat: add background group text sending

[Shinichi1Kudo]

Summary

• Add wechat-cli send GROUP TEXT for background text sending to an exact, uniquely resolved group chat.
• Support macOS ARM64 WeChat 4.1.8, with a pinned binary fingerprint checked before submission so incompatible binaries fail closed.
• Confirm success only after WeChat's Mac-local outgoing message row has a positive local_id, positive server_id, the expected target and sender, exact text, and sent status.

Scope and safeguards

• Group text only. Direct messages, File Transfer, images, files, voice, video, stickers, and other message types are rejected.
• Targets must be either a unique exact group name or a real @chatroom ID already present in the contact database. Fuzzy names, duplicate names, forged IDs, and non-group contacts are rejected before submission.
• The authenticated in-process bridge invokes WeChat's official message-task builder and submit chain on WeChat's main thread. It does not focus the WeChat window, operate the input box, simulate keyboard or mouse input, or write directly to WeChat's database.
• Submission is treated as irreversible. If the submission boundary is crossed but local confirmation is incomplete, the command returns exit code 4 / status: unknown and explicitly disallows automatic retry.

CLI behavior

wechat-cli send "Exact Unique Group Name" "Message text"
wechat-cli send "54597320555@chatroom" $'Line one\nLine two' --timeout 15 --format json

Successful JSON includes success, status, request_id, group, username, local_id, and server_id. A successful result uses status: server_accepted.

Verification

• python -m pytest -q: 279 passed
• Native bridge and fake-host contract compile with -Wall -Wextra -Werror as part of the test suite
• English and Chinese documentation cover supported scope, data flow, local visibility, exit codes, and operational risks