feat: add --path flag for path-based routing

[rqbazan]

Summary

• Add --path <prefix> flag to route multiple apps under one hostname by URL path prefix
• The proxy uses longest-prefix matching to dispatch requests to the correct backend
• Full request path is forwarded unchanged -- backends handle their own base path
• Also available via PORTLESS_PATH env var
• Fix: pass --wildcard flag when auto-starting proxy (was missing from runApp)

portless myapp vite dev                  # serves /
portless myapp --path /api pnpm start    # serves /api/*
portless myapp --path /docs next dev     # serves /docs/*

Useful for local API gateways, microfrontends, monorepos, or any setup where services share a domain.

Implementation Details

• RouteInfo gains an optional pathPrefix field. Route identity becomes hostname + pathPrefix -- same hostname with different paths coexist without conflict.
• findRoute() performs two-stage matching: hostname first (exact, then wildcard fallback), then longest path prefix among candidates. A matchesPathPrefix() helper enforces / boundary checks (/api matches /api/users but not /api-v2).
• --path is recognized in parseRunArgs, parseAppArgs, and auxiliary commands (get, alias, list).
• Backward compatible: routes without --path work exactly as before. Existing routes.json files are read without migration.

Test plan

• pnpm test -- 355 tests pass (35+ new covering path routing, route store, CLI parsing, utils)
• pnpm typecheck -- clean
• pnpm lint -- clean
• pnpm format:check -- clean
• pnpm build -- builds including docs site
• Manual: run three apps under one hostname with different --path prefixes, verify requests route correctly
• Manual: wildcard subdomain + path routing works together
• Manual: PORTLESS_WILDCARD=1 correctly passed on proxy auto-start

[vercel]

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[rqbazan]

Hi @ctate , I wanted to follow up on this PR. It's been a few days and I'd love to keep things moving — would you be able to take a look when you have a moment? Happy to make any changes if needed. Thanks!