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__/server-startup-feedback-export.test.ts

@@ -1,4 +1,10 @@
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const ORIGINAL_PAPERCLIP_API_URL = process.env.PAPERCLIP_API_URL;
+const ORIGINAL_PAPERCLIP_RUNTIME_API_URL = process.env.PAPERCLIP_RUNTIME_API_URL;
+const ORIGINAL_PAPERCLIP_RUNTIME_API_CANDIDATES_JSON = process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON;
+const ORIGINAL_PAPERCLIP_LISTEN_HOST = process.env.PAPERCLIP_LISTEN_HOST;
+const ORIGINAL_PAPERCLIP_LISTEN_PORT = process.env.PAPERCLIP_LISTEN_PORT;
 
 const {
   createAppMock,
@@ -265,6 +271,26 @@ describe("startServer PAPERCLIP_API_URL handling", () => {
     delete process.env.PAPERCLIP_API_URL;
   });
 
+  afterEach(() => {
+    if (ORIGINAL_PAPERCLIP_API_URL === undefined) delete process.env.PAPERCLIP_API_URL;
+    else process.env.PAPERCLIP_API_URL = ORIGINAL_PAPERCLIP_API_URL;
+
+    if (ORIGINAL_PAPERCLIP_RUNTIME_API_URL === undefined) delete process.env.PAPERCLIP_RUNTIME_API_URL;
+    else process.env.PAPERCLIP_RUNTIME_API_URL = ORIGINAL_PAPERCLIP_RUNTIME_API_URL;
+
+    if (ORIGINAL_PAPERCLIP_RUNTIME_API_CANDIDATES_JSON === undefined) {
+      delete process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON;
+    } else {
+      process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON = ORIGINAL_PAPERCLIP_RUNTIME_API_CANDIDATES_JSON;
+    }
+
+    if (ORIGINAL_PAPERCLIP_LISTEN_HOST === undefined) delete process.env.PAPERCLIP_LISTEN_HOST;
+    else process.env.PAPERCLIP_LISTEN_HOST = ORIGINAL_PAPERCLIP_LISTEN_HOST;
+
+    if (ORIGINAL_PAPERCLIP_LISTEN_PORT === undefined) delete process.env.PAPERCLIP_LISTEN_PORT;
+    else process.env.PAPERCLIP_LISTEN_PORT = ORIGINAL_PAPERCLIP_LISTEN_PORT;
+  });
+
   it("uses the externally set PAPERCLIP_API_URL when provided", async () => {
     process.env.PAPERCLIP_API_URL = "http://custom-api:3100";
 
@@ -272,6 +298,10 @@ describe("startServer PAPERCLIP_API_URL handling", () => {
 
     expect(started.apiUrl).toBe("http://custom-api:3100");
     expect(process.env.PAPERCLIP_API_URL).toBe("http://custom-api:3100");
+    expect(JSON.parse(process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON ?? "[]")).toEqual(
+      expect.arrayContaining(["http://custom-api:3100"]),
+    );
+    expect(JSON.parse(process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON ?? "[]")[0]).toBe("http://custom-api:3100");
   });
 
   it("falls back to host-based URL when PAPERCLIP_API_URL is not set", async () => {