fix(railway): root Dockerfile = MCP; Railway uses Dockerfile.web via RAILWAY_DOCKERFILE_PATH

[hwesterb]

Corrects the direction of #39. The right structure is:

• Root `Dockerfile` = MCP (Glama detects it at canonical path — the whole reason the Dockerfile was added in build: add Dockerfile for Glama check + containerized use #37)
• `Dockerfile.web` = web deploy (Railway points at it via env var)

What changed

• `Dockerfile` restored to the original MCP content from build: add Dockerfile for Glama check + containerized use #37 (unchanged from there).
• `Dockerfile.mcp` removed (superseded — the root file is the MCP one now).
• `Dockerfile.web` created with the multi-stage build that's Railway's actual web build. Includes the `.claude-plugin/` and `skills/` COPY fix that would have been PR fix(docker): include .claude-plugin and skills #40 (hatchling's `force-include` needs them).
• Railway env var `RAILWAY_DOCKERFILE_PATH=Dockerfile.web` set via `railway variable set` — this tells Railway to build from the non-canonical path instead of auto-detecting the root Dockerfile.

Why this is the right split

• Glama: expects canonical `/Dockerfile` at the repo root for MCP introspection. We now have it.
• Railway: has an explicit pointer to `Dockerfile.web` via env var that overrides auto-detection. No confusion.
• No env var race: the `RAILWAY_DOCKERFILE_PATH` variable was set on Railway with `--skip-deploys`, so no deploy kicks off until this PR merges.

Test plan

• `railway variables | grep -i dockerfile` shows `Dockerfile.web`
• Deploy on merge builds from `Dockerfile.web`, not root `Dockerfile`
• semahash.org serves the new build
• Glama finds `Dockerfile` at canonical path when they scan (still the MCP content)