docs: add Express-to-Hono migration guide

[PAMulligan]

Summary

Adds docs/migration/from-express.md — a side-by-side migration guide helping Express.js developers transition to Nerva/Hono. Express is the most common starting point for backend developers evaluating Nerva, so a clear, familiar-pattern → Nerva-pattern mapping directly addresses the #1 question new users have.

What's covered

All required topics, each with Express → Hono comparisons:

• App setup and middleware registration (express() + app.use() → new Hono() + app.use("*", ...), with the npm packages Hono replaces built in)
• Route definition (app.get/post/put/patch/delete — near-identical, plus chaining)
• Request/response handling (req.body → await c.req.json(), res.json → return c.json, the full input/output mapping)
• Middleware patterns ((req, res, next) → (c, next), before/after split by await next(), short-circuit by returning, typed c.set/c.get vs req.user)
• Error handling (error middleware → app.onError, throw vs next(err), app.notFound)
• Route grouping (express.Router() → new Hono() + app.route(), group-scoped middleware)
• Static files (express.static → serveStatic, plus the Workers [assets] caveat)
• Environment variables and config (the process.env vs Workers c.env trap, centralized through Nerva's Zod-validated config module)

Plus:

• An Express ↔ Hono cheat-sheet table for quick reference
• A porting-gotchas checklist (forgotten return, async body, etc.)
• A "What Nerva adds on top of Hono" section (10-phase pipeline, 24 agents, 12 skills, templates/deployment targets)

Grounding

Examples mirror the actual generated snippets (templates/snippets/node and cloudflare): Zod-validated config.ts, c.env bindings on Workers, the real middleware stack, app.onError with typed errors, and chained new Hono() sub-apps — so the guide stays consistent with what the generator emits.

Docs index

Linked from the README documentation table.

CI

All internal links resolve to existing files; external links limited to hono.dev, hono.dev/docs/api/exception (already used elsewhere in docs), and expressjs.com to keep lychee link validation green.

Fixes #20

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