Linked sign-in methods read "Link an account you have" and never offer to create one #2883

The other half of #669. Today /signin and /signup show Continue with <host> buttons for the edge login and the parent forge (src/web/pages/site.ts ~lines 260 and 389). Linking from account settings already exists (33dd7fb6).

Done looks like

• [ ] No page under /signin or /signup contains "Continue with". The linked methods sit under the heading "Link an account you have", with one line saying they sign in an account that already exists and never make one.
• [ ] A first sign-in through a linked method, for an identity no account here holds, creates no user. It ends on /signup with a sentence saying how to get an account. If the code path today does create one, close it. Test this at the action level, not only in the markup.
• [ ] Tests: the markup assertion, plus a test that an unknown linked identity leaves the user count unchanged. Both fail on main.
• [ ] Before pushing, run bun x tsc --noEmit, test/signin-edge.test.ts, test/signup-gate-parent.test.ts and the new test.