…or coding agents (wachterjohannes)

This PR was squashed before being merged into the main branch.

Discussion
----------

[Mate] Add skills:install to install extension skills for coding agents

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Docs?         | yes
| Issues        | -
| License       | MIT

Agent Skills (`SKILL.md`) shipped by Mate extensions are now installed onto the
filesystem where coding agents read them — a polyfill until skills can be served
over MCP directly.

### Background — MCP Skills (SEP-2640)

This is the **consumer/polyfill side** of the MCP Skills effort:

- **SEP-2640** proposes serving Agent Skills over MCP's Resources primitive as
  `skill://` resources — modelcontextprotocol/modelcontextprotocol#2640.
- **#2132** already added skill *content* to the Mate Symfony bridge, anticipating
  the extension (agentskills.io + SEP-2640 layout) — #2132.
- **modelcontextprotocol/php-sdk#372** implements the *serving* half in the PHP
  SDK (`skill://` resources + discovery index).

The missing piece is **getting those skills onto the filesystem**: Claude Code and
other agents do not consume MCP-served skills yet (June 2026) — they only read
skills from disk. This PR bridges that gap and can be retired once agents consume
MCP-served skills end-to-end.

### How it works

Extensions declare one or more skills directories via a new `extra.ai-mate.skills` key:

```json
{
    "extra": {
        "ai-mate": {
            "scan-dirs": ["src"],
            "skills": ["skills"]
        }
    }
}
```

Each immediate subdirectory holding a `SKILL.md` is one skill. Mate **symlinks** each one
— under a `mate-` prefixed directory name (e.g. `mate-system-information`) to avoid
clashing with skills you maintain from other sources — into `.agents/skills/` (read by
Codex, OpenCode and GitHub Copilot) and mirrors it into `.claude/skills/` for Claude Code,
which only reads its own directory. The links point into the gitignored `vendor/`, so
skills auto-update on `composer update`.

### Usage

Skills install automatically as part of `discover`, which already runs after
`composer require`, so there's usually nothing to run:

```terminal
$ vendor/bin/mate discover
```

For an explicit re-sync:

```terminal
$ vendor/bin/mate skills:install
```

The core package ships a built-in `system-information` skill to demonstrate the
format and dogfood the mechanism.

### Notes

- Documentation added under `docs/components/mate.rst` and
  `docs/components/mate/creating-extensions.rst`; `CHANGELOG.md` updated.
- Covered by tests (installer, command, discovery, and the `discover` auto-sync);
  PHPStan clean.

Commits
-------

6a88faf [Mate] Add skills:install to install extension skills for coding agents