[Mate] Remove composer plugin and use recipes instead

[wachterjohannes]

After a discussion with Nicolas Grekas, the symfony/ai-mate-composer-plugin package should be removed and its behaviour replaced by a Symfony Flex recipe. This issue tracks the full cross-repo plan — nothing in the plugin can be dropped until the pieces below are in place.

Cross-repo PRs

#	Repo	PR	What	Status
1	symfony/ai	symfony/ai#2027	mate discover --ignore-missing-file	open
2	symfony/ai	symfony/ai#2026	Drop symfony/ai-mate-composer-plugin from symfony/ai-mate's require list (depends on #2027)	draft
3	symfony/flex	symfony/flex#1089	Auto-wire @auto-scripts into post-install-cmd / post-update-cmd	draft
4	symfony/recipes	symfony/recipes#1535	The symfony/ai-mate recipe	draft
5	symfony/ai	follow-up	Delete the composer plugin source	not started

Summary

The plugin (see src/mate/composer-plugin/src/MatePlugin.php) subscribes to Composer's POST_INSTALL_CMD / POST_UPDATE_CMD and:

• prints a "run vendor/bin/mate init" banner if mate/extensions.php is missing,
• otherwise runs php vendor/bin/mate discover --composer and forwards its output (non-zero exit → warning, not failure).

This behaviour is replaced by:

Plugin behaviour	Replaced by
"Run mate init" banner	Recipe post-install-output (symfony/recipes#1535)
Run discover on install/update	Recipe composer-scripts → @auto-scripts (symfony/recipes#1535), wired by Flex (symfony/flex#1089)
Guard on mate/extensions.php existing	mate discover --ignore-missing-file (#2027)

Usage

After all five PRs are merged and released, a fresh composer require symfony/ai-mate reproduces the current plugin's UX on both Symfony skeleton and plain PHP projects — without any custom Composer plugin shipped from the package.

Open points

• Final flag name on Mate (--ignore-missing-file vs alternatives) — currently using --ignore-missing-file in [Mate] Add --ignore-missing-file option to discover command #2027.
• Flex target branch — 2.x or minor bump? — currently 2.x in [ComposerScriptsConfigurator] Auto-wire @auto-scripts into post-install-cmd and post-update-cmd flex#1089.
• recipes vs. recipes-contrib for the recipe location — currently symfony/recipes in [AiMate] Add recipe recipes#1535.

[wachterjohannes]

@nicolas-grekas i have research and put together my findings - i tried to find the sweetspot between heavy lifting in flex and be able to use the package also in "none" flex projects.

the change in flex is really small - i can do that but it would be awesome if you accept the assumptions and the preferred solution