Add VitePress support to framework auto-port-injection

[alistair3149]

Summary

Adds vitepress to the FRAMEWORKS_NEEDING_PORT map so portless auto-injects --port, --strictPort, and --host for VitePress dev servers, matching the existing behavior for plain Vite.

Why

VitePress's CLI ignores the PORT env var and binds to its own default (5173), so the random port portless allocates goes unused and proxy requests return 404. Concretely, in a VitePress project:

$ portless run pnpm exec vitepress dev
PORT=4530 HOST=127.0.0.1 ... pnpm exec vitepress dev
  vitepress v2.0.0-alpha.17
  Local:   http://localhost:5173/      # ignored PORT, bound 5173
$ curl https://myapp.localhost/         # 404 with x-portless: 1

VitePress wraps Vite's dev server. In vitepress/dist/node/chunk-*.js:

async function createServer(root, serverOptions = {}, ...) {
  const { base, ...server } = serverOptions;
  return createServer$1({ ..., server, ... });
}

Parsed argv flows through to Vite's server config unchanged, so --port, --strictPort, and --host all behave the same as they do for plain Vite. SvelteKit-style indirection isn't needed here because vitepress is the actual CLI basename; findFrameworkBasename matches it directly.

Changes

• packages/portless/src/cli-utils.ts: add vitepress: { strictPort: true } to the framework map (one line)
• packages/portless/src/cli-utils.test.ts: add a unit test mirroring the vp test
• packages/portless/src/cli.ts: list VitePress in the --help framework enumeration
• README.md and skills/portless/SKILL.md: list VitePress alongside Vite/VitePlus per AGENTS.md docs-update rules

Test plan

• pnpm -C packages/portless exec vitest run src/cli-utils.test.ts passes (102 tests)
• New injects ... for vitepress command test asserts the exact arg array
• pnpm exec prettier --check passes on all modified files
• Verified empirically with vitepress@2.0.0-alpha.17: without this change, portless allocates a port that VitePress ignores and the proxy 404s; with this change applied locally, the named URL serves the dev server end-to-end

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