feat(plugin): remove plugin execution timeout, forward Ctrl-C to the plugin

[sbaerlocher]

Why

Plugin scripts ran with a hardcoded 300-second timeout that silently killed legitimately long-running plugins (deploys, imports, interactive shells).

A configurable @timeout and a live "still running" status line were both tried and dropped:

• the timeout is a knob nobody wants to tune;
• any in-band status line fights the plugin for the terminal — setTty hands the TTY to the plugin, so dde cannot redraw around its output — and breaks differently across shells / emulators (kept clobbering interactive prompts and leaving cursor artefacts).

What

• No timeout — plugins run unbounded, keeping their own TTY (colours, docker exec -it, read, … work as usual).
• Ctrl-C forwarded via SignalableCommandInterface → Process::stop(), so the unbounded child is not orphaned when dde is interrupted.
• Piped / captured: output forwarded verbatim — scripted and CI usage stay clean.
• No status line, no ANSI, nothing that depends on terminal quirks. The developer keeps the plugin's output in front of them and aborts if it looks stuck.

Tests / docs

• Unit: non-TTY forwards output cleanly (no control chars); subscribed signals; handleSignal stops the running plugin (orphan guard).
• Docs: docs/extending/plugins.md Execution section.
• Local QA green (1286 tests, PHPStan L8, ECS, Rector) in php:8.5-cli container.