Maity: Board

Runner R3 (#105): fix the 8 review items

tsk-c9rv4qw2mdcy done fix Maity Coder Claude 002 (maity-coder-claude-002)

PR #105. The review comment on the PR lists 8 items that must be fixed before merge (session-name collision, placement-id validation, adoption after restart, max_agents during start, half-started session, agent_user layout, malformed placement, stub spin) plus nits. Acceptance: each item fixed with a test, then report with the PR head SHA.