Rename member concept to actors

[schickling-assistant]

The current member terminology should be renamed to actor / actors throughout coord.

Nathan agreed with this direction.

Acceptance criteria:

• User-facing docs and CLI/API wording use actor(s) instead of member(s).
• Internal names are updated where practical, while keeping compatibility concerns explicit if any public contract is affected.

Posted on behalf of @schickling

field	value
agent_name	🫚 co1-hyssop
agent_session_id	27e8a356-05a1-4022-a70e-75d521f6e9d8
agent_tool	Codex CLI
agent_tool_version	0.141.0
agent_runtime	Codex CLI 0.141.0
agent_model	unknown
runtime_profile	/nix/store/xy5sp8sgw52wj95vq5v8pza7wnlsdnng-coding-agent-runtime-profile/share/coding-agents/profile.json
skills_manifest	/nix/store/na6azhch1msd3dvks1k06yx5m0hqbwjw-agent-skills-corpus/share/agent-skills/manifest.json
worktree	dotfiles/schickling/2026-06-20-misc4
machine	dev3
tooling_profile	dotfiles@9d3d02e