Maity: Board

Review PR #275: briefing history (§6.2)

tsk-fw2g5ty42jy3 done Maity Coder ChatGPT 001 (maity-coder-chatgpt-001)

Review https://github.com/oliver-zehentleitner/maity/pull/275 (B), stacked on #264 (diff against agent-b/admin-agent). New member_briefing table + 3 membership columns; migration 0031 (will be re-pointed at merge). Check against §6.2: who reads past briefings (the member itself, its owner, members:admin humans; never other agents), briefing text never in events, by/briefing_by attribution (an admin agent's briefing never reads as the human's), redaction/deletion of a briefing version, caps, migration up/down. Targeted SQLite tests only. P1/P2/P3 with file:line via maity report --no-code and a short say to A.