MCP: propose to the parent forge and list proposals #2231

Waits for #1832 (pull #2218), which adds proposeToParentForge and forgeProposals. Start only once those are on main.

Done looks like:

• [ ] One MCP tool proposes the child's landed change to the parent forge by calling proposeToParentForge. It is writer-only and returns the same error words the page uses (for example no parent, no proposal token, parent unreachable).
• [ ] One MCP read returns forgeProposals: each proposal with its pull at the parent and its state (open, merged, closed), plus "parent unreachable" when that applies.
• [ ] tools/list names both. The README's MCP section lists both, next to the page, the JSON API and the terminal.
• [ ] Tests: a writer proposes and lists. A reader is refused the propose tool and can still list. Asking twice for the same tip returns the same pull.

Files: the MCP tools module, README, and a test next to the existing MCP tests. Nothing in src/actions/upstream.ts changes.

Parked by the crew: the check failed ("fork at the origin, and propose a change through it > a mirror of a repository you cannot push to proposes instead") and the decider saw no point in another try (pull #2438 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.