Make slic compatible with PHP 8.5

[dpanta94]

Make slic compatible with PHP 8.5, with a PHPCompatibility check in CI to keep it that way.

Ticket: none. This came out of the main → 3.0.0 merge cleanup, not a tracked issue.

What changed

PHP 8.5 deprecates the backtick operator, case labels ending in a semicolon and curl_close(), all of which slic used, so every command printed deprecation notices. Those are replaced. The new sniff also caught slic_stacks_resolve_from_path() reading its argument through func_get_arg(), which since PHP 7 returns the current value rather than the original, so its fallback for paths that do not exist was working with false.

composer compat runs PHPCompatibility (its develop branch, which is where the 8.5 sniffs live) with testVersion 8.2-8.5. The upper bound is pinned because develop already sniffs unreleased 8.6 changes; raise it when a new PHP version ships. The test matrix now covers PHP 8.2 through 8.5 and runs the compat check on 8.5.

Proof

Before, on PHP 8.5.10, any command:

Deprecated: Case statements followed by a semicolon (;) are deprecated ... src/docker.php on line 149
Deprecated: The backtick (`) operator is deprecated, use shell_exec() instead ... src/slic.php on line 1743

After:

composer compat    →  8 / 8 (100%), no errors or warnings
vendor/bin/phpunit →  OK (16 tests, 24 assertions)

Covered by the new PhpDeprecationsTest, which boots the CLI and fails on any Deprecated: output. It was red before the fixes and green after. The func_get_arg fix has no test of its own.

AI usage: Drafted with Claude Fable 5.1. I reviewed and tested every change — this is my work and I am accountable for it.

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