Public projects like GitHub: private by default, public readable by everyone #322

A project is private or public, as on GitHub. Private: owner, members and the organization read and write. Public: everyone in the world reads everything shared, without an account; writing stays with members, outsiders propose.

Today

• Project.IsPublic exists, but only as "Enable public contributions" in the project settings, and only with a verified remote. Semantics: signed-in non-members may read and may create public memories that go to review.
• Every web page carries [Authorize], including Explore. Nobody reads anything without an account.
• MCP: project_create and project_update have no public flag; project_list does not report it. A key of a non-member reaches a public project only through the memory visibility rules.
• Memory visibility public is tied to IsPublic (#321 replaces it with shared).

Decide first, record on #309 or a concept page

1. Verified remote as a precondition? GitHub needs none. Proposal: public needs no remote; a verified remote only marks the repository link as verified.
2. What outsiders see: shared memories, tasks, pages, instructions, history and proposals. Private memories stay with their author. Assignee and agent display names become visible; state this in the confirmation.
3. Search engines: noindex by default with an opt-in per project, or indexed like GitHub.
4. Making a project private again: outsider proposals pending at that moment, forks that recall the upstream (#309: upstream recall takes shared memories of a public upstream), and outsiders' bookmarks. Proposal: pending outsider proposals stay decidable, forks stop receiving upstream recall, links return "not found" as for any private project.
5. Confirmation before going public: a dialog naming what becomes visible and reminding of the guide rule that credentials, account names, security details and legal matters do not belong in a project that may become public. Owner only, not moderators.

Work

• Web: anonymous read of public projects (Explore, project page, memories, tasks, docs, item pages, history) with every action hidden; sign-in prompt where an action would need it. Rate limiting for anonymous reads.
• MCP: project_get reports public; project_store takes public (owner only); any key reads a public project and its shared items; a write from a non-member becomes a proposal, not a refusal. recall on a public project works for any key.
• Live updates (LiveUpdatePoller) respect the same rule for anonymous viewers.
• Explore lists public projects for everyone; the project's public badge and the memory hint from #321 ("shared is visible to everyone in a public project") are shown.
• Setting: "Private" / "Public" with the confirmation from decision 5; the remote precondition removed or kept per decision 1.
• Guide and server instructions: what public means for stored content, in one sentence.

Acceptance

• Signed out, a public project's shared memories, tasks, pages, instructions and their history are readable in the web; a private project returns not found. Both tested.
• A key without membership reads a public project over MCP, and its store becomes a proposal; on a private project it gets not found. Tested.
• Private memories of a public project are never visible to anyone but their author, signed in or out. Isolation tests extended.
• Switching public to private hides everything from outsiders at once; pending outsider proposals remain in moderation.
• The confirmation dialog appears before a project goes public and names what becomes visible.
• No member email address is visible to anyone outside the project.