Fix SSH callback URL selection for LAN and private networks (#4799)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Agents can run on remote hosts via SSH environments
> - When a remote agent needs to call back to the Paperclip API, it
needs a reachable URL
> - But the runtime API URL candidate builder did not account for
private network topologies where the server is only reachable via LAN or
VPN addresses
> - Agents on SSH hosts were failing to connect because the callback URL
pointed to localhost or an unreachable address
> - This PR fixes callback URL selection to honor `PAPERCLIP_API_URL`,
prefer LAN-reachable candidates, filter unreachable link-local
addresses, and include interface hosts in onboarding invite URLs
> - The benefit is SSH-based agents can reliably reach the Paperclip API
on private networks without manual URL configuration

## What Changed

- `runtime-api.ts`: Added `PAPERCLIP_API_URL` as a first-priority
candidate in `buildRuntimeApiCandidateUrls`; extracted
`collectReachableInterfaceHosts` to enumerate non-loopback,
non-link-local network interface IPs with IPv4 preference
- `server/src/index.ts`: Export `PAPERCLIP_API_URL` from the server
environment so it is available to callback candidate resolution
- `server/src/routes/access.ts`: Include LAN interface hosts in
onboarding invite connection candidates
- `server/src/config.ts`: Attempted auto-allowing LAN interface hosts,
then reverted to the per-instance allowlist approach (both commits
included for history clarity)

## Verification

- `pnpm test` — all existing and new tests pass, including new tests for
LAN candidate ordering and link-local filtering
- `pnpm typecheck` — clean
- Manual: start a Paperclip server on a machine with a LAN IP, create an
SSH environment pointing to another host on the same LAN, verify the
agent's callback URL uses the LAN IP rather than localhost

## Risks

- Low-medium. The candidate list now includes more addresses (all
non-loopback LAN interfaces). These are candidates for the agent to try,
not an allowlist — the server's allowed hostnames still gate which
origins are accepted. Ordering change (LAN preferred over loopback)
could affect existing setups where localhost was intentionally
preferred.

## Model Used

Codex GPT 5.4 high via Paperclip.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

server/src/__tests__/invite-onboarding-text.test.ts

@@ -1,4 +1,6 @@
+import os from "node:os";
 import { describe, expect, it } from "vitest";
+import { vi } from "vitest";
 import type { Request } from "express";
 import { buildInviteOnboardingTextDocument } from "../routes/access.js";
 
@@ -114,4 +116,68 @@ describe("buildInviteOnboardingTextDocument", () => {
     expect(text).toContain("Message from inviter");
     expect(text).toContain("prioritize flaky test triage first");
   });
+
+  it("includes LAN candidates when the advertised host is tailnet-only", () => {
+    const networkSpy = vi.spyOn(os, "networkInterfaces").mockReturnValue({
+      en0: [
+        {
+          address: "fe80::1",
+          family: "IPv6",
+          internal: false,
+          netmask: "ffff:ffff:ffff:ffff::",
+          cidr: "fe80::1/64",
+          mac: "00:00:00:00:00:00",
+          scopeid: 1,
+        },
+        {
+          address: "192.168.6.178",
+          family: "IPv4",
+          internal: false,
+          netmask: "255.255.252.0",
+          cidr: "192.168.6.178/22",
+          mac: "00:00:00:00:00:00",
+        },
+      ],
+      utun0: [
+        {
+          address: "203.0.113.42",
+          family: "IPv4",
+          internal: false,
+          netmask: "255.255.255.255",
+          cidr: "203.0.113.42/32",
+          mac: "00:00:00:00:00:00",
+        },
+      ],
+    });
+
+    try {
+      const req = buildReq("paperclip.example.test:3103");
+      const invite = {
+        id: "invite-4",
+        companyId: "company-1",
+        inviteType: "company_join",
+        allowedJoinTypes: "agent",
+        tokenHash: "hash",
+        defaultsPayload: null,
+        expiresAt: new Date("2026-03-05T00:00:00.000Z"),
+        invitedByUserId: null,
+        revokedAt: null,
+        acceptedAt: null,
+        createdAt: new Date("2026-03-04T00:00:00.000Z"),
+        updatedAt: new Date("2026-03-04T00:00:00.000Z"),
+      } as const;
+
+      const text = buildInviteOnboardingTextDocument(req, "token-999", invite as any, {
+        deploymentMode: "authenticated",
+        deploymentExposure: "private",
+        bindHost: "0.0.0.0",
+        allowedHostnames: ["paperclip.example.test", "203.0.113.42"],
+      });
+
+      expect(text).toContain("http://192.168.6.178:3103");
+      expect(text).not.toContain("http://[fe80::1]:3103");
+    } finally {
+      networkSpy.mockRestore();
+    }
+  });
 });