Channel participation and access policy

[dcellison]

Parent epic: channel settings and shared channel authority.

Problem

Standing participation is already channel-wide, but Channel Settings does not present a coherent participation/access policy. Membership and agent controls exist elsewhere without a clear statement of who may change shared channel behavior.

Scope

• Represent standing-participation enablement in the canonical Channel Settings projection while retaining the existing standing authority.
• Define the effective permissions for channel owner, workshop administrator, and participant for metadata, membership, agent attachment, standing participation, workspace grants/binding, and archival.
• Add an Access section that clearly presents the current membership model and management permissions.
• Evaluate private versus discoverable channels against the existing membership model. Implement discoverability only if it can remain explicit, non-enrolling, and fail-closed; otherwise record it as a separately scoped follow-up rather than inventing partial behavior.
• Keep member addition/removal in the People section and agent attachment/detachment in the Agents section; Channel Settings may explain or link to those controls but must not duplicate them.
• Ensure policy changes converge across clients and cannot wake agents or create membership as a side effect.

Acceptance criteria

• Every shared setting has a documented actor/permission rule enforced server-side.
• Non-owners see an accurate read-only view without inert controls that imply authority.
• Standing participation continues to use its existing canonical records and behavior.
• No access-policy operation leaks private channels, adds members, or attaches agents implicitly.
• Multi-principal authorization tests cover every mutation class.