docs: add Express-to-Hono migration guide (#20) (#124)

Add docs/migration/from-express.md mapping every common Express pattern
to its Hono/Nerva equivalent: app setup and middleware registration,
route definition, request/response handling, middleware patterns,
error handling, route grouping, static files, and environment/config.

Includes an Express to Hono cheat-sheet table, a porting-gotchas
checklist, and a section on what Nerva layers on top of Hono (pipeline,
agents, skills, templates). Examples are grounded in the generated
Node and Cloudflare snippets (Zod-validated config, c.env bindings,
app.onError with typed errors, chained Hono sub-apps).

Link the guide from the README documentation index.

Fixes #20

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: PAMulligan

README.md

@@ -200,6 +200,7 @@ Nerva is the backend counterpart in a family of Claude Code-integrated developme
 | Document | Location | Description |
 |----------|----------|-------------|
 | Quickstart | [`docs/onboarding/quickstart.md`](docs/onboarding/quickstart.md) | Get your first API running |
+| Migrate from Express | [`docs/migration/from-express.md`](docs/migration/from-express.md) | Express → Hono cheat sheet and porting guide |
 | Architecture | [`docs/onboarding/architecture.md`](docs/onboarding/architecture.md) | System architecture overview |
 | Troubleshooting | [`docs/onboarding/troubleshooting.md`](docs/onboarding/troubleshooting.md) | Common issues and solutions |
 | Pipeline guide | [`docs/schema-to-api/README.md`](docs/schema-to-api/README.md) | 10-phase pipeline deep dive |