Add multiplexed hostname routing

[RhysSullivan]

AI generated PR disclosure

Screen.Recording.2026-04-29.at.12.14.13.AM.mov

Overall pretty happy with this, behavior:

• If just one app is running, it works like normal
• If multiple apps are running, it prompts you to select and app and saves your choice
• There's a button in the bottom right to swap between selected apps and clear selection
• Worktrees redirect you to the right location

Use case is for redirects with auth. Manually reviewed the code and am happy with it, the HTML insertion is ugly but works.

AI Summary below:

Summary

• add multiplexed hostname routing behind --multiplex / PORTLESS_MULTIPLEX=1
• show a selector when multiple apps share the same hostname and persist the selected app in a cookie
• inject a small app switcher into HTML responses when multiple apps are available
• update route persistence, CLI behavior, docs, and tests for multiplex mode

Validation

• pnpm --filter portless build
• pnpm --filter portless type-check
• pnpm --filter portless lint
• pnpm --filter portless test -- src/routes.test.ts src/proxy.test.ts src/cli-utils.test.ts src/cli.test.ts
• pnpm --filter portless test

Notes

• Manually tested two apps on mux.localhost:1355 with the portless proxy in multiplex mode.

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