fix(railway): remove dead root railway.json hijacking template builds

[AbirAbbas]

Summary

Follow-up to #522. Verified that the repo-root railway.json is dead weight and the actual cause of the Railway template regression — removing it (and the now-redundant per-example file added in #522).

What's actually broken

Every fresh deploy of the controlplane-template Railway template (e.g. the github-buddy project created today) fails on the example node service with:

failed to compute cache key: "/control-plane/web/client": not found

Root cause

Verified across the published Railway templates:

Template	control-plane source
controlplane-template	image agentfield/control-plane:latest
sec-af	image agentfield/control-plane:latest
swe-af	image agentfield/control-plane:latest
contract-af	image agentfield/control-plane:latest
cloudsecurity-af	image agentfield/control-plane:latest
agentfield-deep-research	image agentfield/control-plane:latest

Every control-plane service uses a prebuilt image. Verified the same across every live control-plane deployment in the workspace (9 projects). Other agent services in those templates source from their own repos (sec-af, swe-af, etc.), not from Agent-Field/agentfield. The deployment guide (deployments/railway/README.md) instructs image-based deploys only.

The only service in the entire footprint that sources from Agent-Field/agentfield is controlplane-template's example node, with rootDirectory: /examples/ts-node-examples/init-example and no explicit railwayConfigFile. Railway falls back to repo-root /railway.json, which says "build deployments/docker/Dockerfile.control-plane". That Dockerfile expects the repo root as build context (it COPYs control-plane/web/client/...), but Railway scopes the context to the service's rootDirectory — where control-plane/ doesn't exist. Boom.

Why now

PR #151 (Jan 21) switched the template's control-plane to a prebuilt image and removed railway.toml, but kept railway.json "just in case." That same PR explored a per-example railway.toml override (commit 0ba92fbf — "Add railway.toml to init-example to override root config"), then reverted it (6d71540f) when the image switchover was deemed sufficient. It wasn't — the example node service was still building from the repo, just with no local override. The bug was latent until someone deployed the template fresh today.

Fix

• Commit 1 — delete repo-root railway.json. Nothing depends on it, and its presence breaks the example node service.
• Commit 2 — drop the per-example railway.json from fix(examples/init-example): pin Railway build to local Dockerfile #522. With root config gone, Railway falls through to auto-detecting the local Dockerfile in the rootDirectory — which is what the example was always meant to use, and what the existing local Dockerfile provides (simple node:20-alpine + npm start).

Test plan

• Trigger a redeploy of the example node service in the github-buddy Railway project against this branch and confirm the build uses examples/ts-node-examples/init-example/Dockerfile (not Dockerfile.control-plane) and succeeds.
• Confirm the agent registers with the control plane and shows healthy.
• Smoke-test by deploying the controlplane-template template fresh in a throwaway project and verifying all 3 services come up.