chore: move Railway service config to Config as Code

[CountableNewt]

Railway stops reading railway.json-style config files on 2026-12-01. This moves the Gateway and Web settings from railway/gateway.json and railway/web.json into .railway/railway.ts.

What's in .railway/railway.ts

• Source: GitHub dev in Development and main in Production (via ctx.isEnvironment), Wait for CI on, root directory /.
• Build and deploy settings carried over exactly from the JSON files: Gateway uses the Dockerfile with /health and a 600s healthcheck timeout; Web uses Railpack with Turbo, bun --cwd apps/web start and /.
• Postgres, its volume, and every service variable (as preserve()). railway config apply deletes anything undeclared: the first draft planned to delete the dev database and 29 variables. Values stay in Railway and never enter source.
• Domains are not managed, so the Gateway's runtime-provisioned tenant domains (e.g. mcp.samclemente.me) are safe.
• Adds the railway SDK (3.11.0) as a root dev dependency, which the CLI needs to evaluate the file.

Current plans (read-only)

Environment	Plan
dev	0 add, 4 change, 0 destroy: build and deploy fields only (currently supplied by the JSON files)
production	0 add, 5 change, 0 destroy: the same, plus Gateway rootDirectory /services/mcp-gateway → /

⚠️ Merge order (draft until done)

The services still point their "Railway Config File" setting at /railway/*.json, which this PR deletes.

1. railway link --project 3647f696-1766-459a-ac3f-0482e5a1f26c --environment dev && railway config plan && railway config apply
2. Repeat for --environment production, checking the plan still shows 0 to destroy.
3. In each environment, clear the Config File path for Gateway and Web (Service → Settings).
4. Mark ready and merge. The resulting dev build is the first build fully driven by .railway/railway.ts.

Other changes

• Drop the jq config check from scripts/ci.sh; update the (currently unused) path filters in scripts/ci-detect-changes.sh, railway/README.md and the test-plans doc.

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