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__/paperclip-env.test.ts

@@ -30,14 +30,14 @@ afterEach(() => {
 
 describe("buildPaperclipEnv", () => {
   it("prefers an explicit PAPERCLIP_RUNTIME_API_URL", () => {
-    process.env.PAPERCLIP_RUNTIME_API_URL = "http://100.104.161.29:3102";
+    process.env.PAPERCLIP_RUNTIME_API_URL = "http://203.0.113.42:3102";
     process.env.PAPERCLIP_API_URL = "http://localhost:4100";
     process.env.PAPERCLIP_LISTEN_HOST = "127.0.0.1";
     process.env.PAPERCLIP_LISTEN_PORT = "3101";
 
     const env = buildPaperclipEnv({ id: "agent-1", companyId: "company-1" });
 
-    expect(env.PAPERCLIP_API_URL).toBe("http://100.104.161.29:3102");
+    expect(env.PAPERCLIP_API_URL).toBe("http://203.0.113.42:3102");
   });
 
   it("falls back to PAPERCLIP_API_URL when no runtime URL is configured", () => {
@@ -52,6 +52,7 @@ describe("buildPaperclipEnv", () => {
   });
 
   it("uses runtime listen host/port when explicit URL is not set", () => {
+    delete process.env.PAPERCLIP_RUNTIME_API_URL;
     delete process.env.PAPERCLIP_API_URL;
     process.env.PAPERCLIP_LISTEN_HOST = "0.0.0.0";
     process.env.PAPERCLIP_LISTEN_PORT = "3101";
@@ -63,6 +64,7 @@ describe("buildPaperclipEnv", () => {
   });
 
   it("formats IPv6 hosts safely in fallback URL generation", () => {
+    delete process.env.PAPERCLIP_RUNTIME_API_URL;
     delete process.env.PAPERCLIP_API_URL;
     process.env.PAPERCLIP_LISTEN_HOST = "::1";
     process.env.PAPERCLIP_LISTEN_PORT = "3101";