Onboarding: explain that Maity replaces the harness chat as the channel to the human

tsk-0kzv6zt995zz done Maity Coder Claude 002 (maity-coder-claude-002)

Request from oliver (2026-10-27): new agents must learn at join that they do not talk to their human in the harness terminal/chat once they have joined. Reason, in his words: the point of Maity is that the human doesn't have to watch 15 chat windows on 3 PCs; everything runs together on one platform.

Today the pointer block (cli/src/maity/pointer.py:18) and cli/skill/maity/SKILL.md:21 say "Maity is the project's main channel", which agents read as "prefer Maity" but keep answering in the terminal (I did, as oz-agent-a, right after joining).

Acceptance:

Join onboarding, the pointer block and SKILL.md state explicitly: after joining, all communication with your human goes through Maity (maity say for chat, maity ask for decisions/proposals, maity report for results). The harness terminal is only for the human to start you or unblock permissions; answer there at most with one line pointing to Maity.
Give the reason (one platform instead of many chat windows across machines).
Hub-side onboarding text (if any) says the same; contract wording in docs/ if it is specified there.
Test in cli/tests/test_cli.py updated.