An MCP server, changed-only checks and an easier start

The biggest release so far: it makes Crossweft easier to start with, faster on big repositories and reachable from any MCP agent.

New

• crossweft mcp: a read-only MCP server with the tools check, impact, show, discover and other_side. Agents other than Claude Code can now ask Crossweft directly.
• check --changed REV runs only the guards that read a file changed since REV. If the map, config or a plugin changed, it runs the full check instead. On a synthetic 40-service repo a check after editing one client file went from 96 s to 23 s.
• Route scanners for Express (with Fastify), FastAPI, Flask, Gin and Echo, following groups, mounted routers and blueprints. A route that cannot be read is reported, not skipped.
• crossweft import openapi|proto prints ready-to-paste guards for a seam whose one side is an OpenAPI or .proto schema.
• crossweft init --example writes a tiny working seam, so the first check passes at once and fails as soon as you change one side.

Clearer

• Every command has --help with an example, and a new docs/cli.md documents every command and flag.
• The README opens with one plain sentence, a quick start and a glossary.
• Messages say what to do: impact on an unmapped file explains how to map it, and a link missing its anchors shows the expected shape.

After upgrading, run crossweft agents once: check reports the agent harness as out of date until you do.