Join public projects so they appear in your own project list #350

A person or agent who is not a member can take part in a public project (#322): join it, see it in the sidebar and the project list, and leave it again. Participation is what puts a public project into your lists; reading alone does not.

Today

• KnowledgeService.Projects returns every project with IsPublic for every signed-in user, so each public project appears in everyone's sidebar and home list. With real public projects (#322) that floods the lists; it must change together with this task.
• Access is owner, ProjectMembers (reader/contributor/moderator, added by a moderator by email) or organization membership. There is no self-service way in.

Decide first (record on the #322 concept page)

1. What joining is: a self-service ProjectMembers row with a new role (for example participant: read plus proposals, like an outsider, but listed), or a separate follow table like ProjectFavorites. Proposal: a membership role, so permissions, assignee lists and exports keep one source of truth.
2. Rights of a participant compared with an outsider of a public project: may they be assigned tasks, claim ready work, write directly or only propose?
3. Leaving: always possible; what happens to their claimed tasks and pending proposals.
4. Agents: can a key join on its owner's behalf (MCP project_store or item_action), or only people in the web?

Work

• Join / Leave on a public project's page (and on Explore); the project then appears in the sidebar, the home list and project_get.
• Projects() lists owned, member, organization and joined projects only; Explore stays the place to find all public ones.
• Favorites (#ProjectFavorites) and Recent keep working for joined projects; leaving removes the favorite.
• Live updates so the sidebar follows a join or leave at once.

Acceptance

• A public project you have not joined is readable but not in your lists; after joining it is, after leaving it is not.
• A private project cannot be joined; the request answers not found.
• Participant rights match decision 2 and are tested over the web and MCP.
• The smoke test (#345) or an HTTP test covers join and leave.