Add background app management

[bjesuiter]

Human Note

Hey there, Benjamin here!
Basic reason for this pr:
I code heavily with agents and was surprised that i cannot run a portless frontend process in background right now.
So I ported a background process management cli subcommand, called bg, into the protless cli, inspired by bgproc: https://github.com/ascorbic/bgproc.

This change wouuld allow me to go from this:

bgproc start -n web -- portless run -- pnpm dev

to this

# reading it's settings from package json and having stop, restart and logs commands available
portless bg start

I'm open to any changes to better fit this into portless philosophies if i missed one. I tried to be as close to the shape of the existing portless cli as possible.

Disclaimer: This PR is heavily Coding Agent assisted, but the structure and product decisions where made by me.

Summary

Clanker text from here on

Adds first-party background app process management under portless bg.

• adds bg start, stop, restart, status, list, logs, and clean
• backgrounds portless run so existing route, proxy, TLS, sharing, and app command behavior is reused
• stores bg registry and capped stdout, stderr, and lifecycle logs in the active Portless state dir
• adds docs, CLI help, clean integration, and bg help tests

Design notes: bjesuiter#1

Validation

• pnpm --filter portless type-check
• pnpm --filter portless lint
• pnpm --filter portless build
• pnpm --filter portless test
• manual bg start, status, logs, stop smoke test

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